If I'm redshirting a freshman, is there any real need to give him study hall minutes?

The only downside I can see if that his GPA may start out very low next season, and I may need to give him more SH minutes than I would otherwise to keep him eligible, but that seems like a small price to pay to get him more development during his redshirt season.

I haven't redshirted anyone in a LONG time, but I assume they'd need the normal SH minutes to stay eligible. It may cost a few extra minutes but going ineligible will cost you a lot more.

I typically give my redshirted players far fewer study hall minutes than a typical player. Just enough to keep them safely above 2.0 so I don't start out too far in the hole next season. I agree with you that it's worth it for the extra skill development.