The first thing to do is to look critically at the sources and keep in mind the following basic notions: 1) the lore is not a revealed truth, but a poetic take on reality and the Gods; 2) the existing Old Norse lore was generally not recorded by pagans, but by Christians with their own mindsets and agendas; 3) the nonexistence of an orthodoxy in pre-Christian Old Norse religion, the basis on oral traditions and the diversity of communities throughout Scandinavia, with their own needs and experiences, provides a fertile ground for diverse versions of myths, stories and godly roles. And as if this wasn’t enough, 4) what lore we have is not a mythological corpus of left-alone pagans in some valley in northern Europe, but a very limited group, geographically and chronologically, and from a period of time when pre-Christian religions were facing opposition from Christianity.
Secondly, one should never forget that, in line with other European pre-Christian religions, Old Norse one was most likely orthopraxic, i.e., its focus was on correct ritual worship, not doctrine. It doesn’t mean that there weren’t traditional beliefs, just that it was open to personal gnosis and had no central authority sanctioning what was and wasn’t an admissable belief. It also means that there was no religious morality, rather a social one enshrined by religion. For instance, the social virtue of hospitality, highly relevant in a time of sparse settlements and no mass media, was protected by the Gods’ example, as in Odin’s advises in Hávamál or in the tale in Grimnismál.
Thirdly, there’s this idea that the world – ours and the Gods’ – is divided in two opposite sides, one good and another evil, the former being worthy of worship while the latter is not. So the Jotnar are evil, they should not receive gifts (except for those who joined the divine ranks, of course…) and Ragnarok leaves no room for neutrality: if you’re not with the High Ones, you’re against them. Siegfried Goodfellow even goes as far as suggesting that worship requires worthiness, assumingly moral one given his critiques of Loki (Whom he considers to be unworthy). But this implies setting aside something which was a natural part of pre-Christian religions: apotropaic worship, i.e., offerings presented not to ask for blessings but to drive away a given deity. To put it in a practical example, “regular” sacrifices could call on Thor for a safe sea journey while apotropaic would mean the giving of something to Ran to prevent Her from taking seamen’s lives. And in Scandinavia, among the Saami, there was the practice of sacrificing to a deity of disease to cure people: appart from calling on a Goddess of healing, one could also “bribe” that of illness to go away. This means that worship in pre-Christian religions may well have had nothing to do with moral worthiness, but merely practical one. The idea that “evil” should not be worshipped owes more to Christianity than to the original European pagans, who did not see things in strict tones of black and white.
So where does this leave Loki? In an ambigious position, as is proper to Him. He’s not the Norse devil or the fallen angel who’s responsible for all the evil in the world: just because medieval Christian authors saw things in terms of black and white – and had no problem in showing it in their recording of old Norse myths – doesn’t mean modern heathens should too. Plus, given the expected diversity in an orthopraxic polytheistic religion, the fact that a group of pagans more linked to God A or B had a given set of beliefs and principles doesn’t mean Loki wasn’t seen differently by other heathens. He’s above all a Trickster: while Odin has a thing for wisdom and his beasts feast on the battlefield, Thor feels at home fighting, Freya loves lust and Freyr the growth of the fields, Loki’s cup of tea is humor, often chaotic but often too with a final practical twist. He walks the thin line between social rules and their breaking at the dawn of new things; He’s the agent of dramatic change, dramatic both in process and price, but which nontheless makes way for something new and shinny. Who caused the chain of events which led to the making of the Gods treasures (Draupnir, Gullinbursti, Skíðblaðnir or the famous and vital Mjöllnir)? Loki did! It doesn’t mean that He’s a pleasant deity to keep you company, but neither are the Gods of the underworld or of disease. And yet, ancient pagans would pay Them homage whenever they wanted to keep Them away or ask Them to do something drastic: curses, destruction of an enemy or even the request for a drastic solution to a difficult problem. Is it dangerous to do so? Yes! But that’s how things usually work with dark or wild powers and Loki just happens to be one of those.
So, in short, I do think the Trickster has a place in heathen worship, I do not see Him as a Norse devil, even though that doesn’t mean He’s a pleasant fellow. But as is natural in a polytheistic religion, there are many things above, around and bellow us, not all of them friendly or pleasant to deal with, and most have their own ways, rules and mindsets. Doesn’t mean they should be left out of the offerings table (even if you may put Them at a different table, for safety reasons).