[Kevin Sevcik]

*dons fire retardant suit and climbs into bunker to shelter from the coming holy war*

This issue is one of high contention in the FIRST community. You'll find teams that run the gamut from 100% student built to 100% engineer built and everything inbetween. So to answer your original question, any level of engineer involvement is "acceptable."

I think the real question here is how much involvement is right for your team. Every team is different, and everyone on that team have different reasons for being there. I think the most productive thing you could do in this area is to sit down with your teammates, mentors, and sponsors if at all possible and have a nice long discussion about why you're all their and what you want out of it. Especially the sponsors, they pay the money, so they should have some say. If you can get in touch with a high up person in any company that sponsors you, I think you'll find they have rather well considered opinions on the issue.

In any case, all that matters is that your team is happy with the amount of involvement. You're there to be inspired and possibly learn something about engineering. This can't happen if large parts of the team (students or mentors) are sulking because of this arrangement.