In Piccard, is it best when tagging songs that feature multiple artists, to include them in the artists field or not ? i.e Snoop dog (feat. xxx) or 2Pac & Snopdog
Some of my songs are multiple artists,while others don’t, so should i rely on what MB see them as or should i trust the original release where i got them from? (e.g CD etc) ?
These were not submitted by me, i’m just tagging them.
I mean, it’s your collection, so it just depends on how you like it. I generally include them, but there are plugins to move them to titles or just get rid of them if you prefer that at Available Plugins - MusicBrainz Picard 
Stuff you do in Picard doesn’t modify anything in MusicBrainz.
If you want to set up track names for a release MB doesn’t have (so that other people benefit from that information), you need to do that through the website, not Picard. If you do so, then you need to follow MB’s rules of course (so you’d need to use artist credits for A feat. B instead of having (feat. B) in the title.