Every project I do I turn in the multitracks, with no automation/samples/plugins free of charge. I pretty much have to, as it's what's expected. IF they've paid for the recording, then THEY OWN IT. You're not required to give them an entire breakdown of what you did, but you are required to hand over any and all raw files they request.
The stems issue isn't a big deal for me either, cause I always print a bunch of mixes for every song (main/inst/acca/vox up/vox dwn/tv/drums only/guitar/bass/keys/other/vox).
The whole "how long to hold onto it for" is why I do this. I don't want backups for years or even months. When I hand over the drive I said to them "this is ALL the files, in 48 hours I'm deleting it off my drive. It's your responsibility now".
Of course bands are fucking idiots, so I generally hold onto a project for about a month to 6 weeks. Then after that period, I delete everything but the final mix session, and put it in my archive. That archive gets cleaned out about once or twice a year.
The biggest thing for engineers is to not become the archive for bands. Let them know that it's on them...it's their job, not yours, to make sure there files are available should they need em. If you hold onto the multitracks, you're putting yourself in a dangerous position where you have assumed responsibility of the project for an infinite amount of time. 99% of the time, no band will ever request files after, but if they do and you don't have em...you're fucked.