I would say it depends on how it fits into your storyline. The setting and background I have been tasked with writing for another RPG- and the contract also stipulated a minimum of three novels should they option them- is based directly in existing ancient legends and lore. So, I have to do extremely detailed and extensive research into those pre-Christian religions and cultures that I am unfamiliar with (since the setting involves several European, African, Asian, Oriental, American- north and south- and Australian indigenous religions, and my expertise is on the European ones) and get it right. The last thing I want is a ton of nastygrams from thousands of angry Hellenes in Greece because I screwed up something basic about the Olympian Gods. If the core of the storyline is not based in a specific mythology or requires precise accuracy, get creative. A good example of a blend of legends throughout history that has been "tweaked" with significant creative license is Feist's
Fairy Tale.
A second point to remember- and I touched on this a bit above- is that a lot of those ancient religions and their attendant mythologies are STILL practiced today, and many of the followers of those ancient faiths- or their derivatives- are likely to be in your target audience. Granted, not all of them are thin-skinned purists who will show up on your doorstep if you bugger up a deity's name. Hell, I went to see Thor with a couple of Theodisc (Norse/Germanic heathens to whom Thor IS one of their Gods) fully expecting them to burn the theater to the ground and blood-eagle somebody. I was surprised, they loved it (granted, largely because it showed their Gods in a pretty positive light, and never specifically said they
weren't Gods), so if you get the gist right and are flattering or fair, they will likely let the details slide. I know a lot of Ásatrú and Theodisc Dresden fans who
actually cheered the portrayal of Valkyries and Odin in the last two novels
, so there's room for fudging without irking the followers of the indigenous faiths. Now that he
has introduced the Fomóirí into the mix, I am a bit on pins and needles to see how he will treat the Gods of my people, the Túatha de Dannan, since the Fomóirí are their ancient enemies (when they weren't knocking each other up, at least with the pretty ones out of the Fomóirí) and a core part of our faith (Sinnsreachd).
So, you can even hook readers in by working into it little by little over the course of a novel or series of novels.
My $0.02 anyway