I'm cashier at a menards in Wisconsin, 8years now. So I can be of help here.
My store price matches with an ad, you can also use MFC's with a price match AND use a rebate check. I have worked at two different stores: neither of them ever kept a guests rebate before it was used; we cross off the total and write the remaining balance. What store does that? I'd like to ask my manager to see if its a per-store policy
We also have a barcode in our cashiers book to scan if a MFC coupon doesn't scan, a manager will verify it's okay and this way you still get your savings!
There are VERY FEW rebates that can come as a real check you'd take to your bank; I've only seen about 4 of these in 8years, so you can expect an in store credit for the majority of rebates.
People abuse the rebate limits, they send extras to friends or family members to get more that what's allowed per household and when I catch people I write on their rebate for the next cashier to "Check ID." That's just something I do personally, because I'm a stickler with limits and greed.
If you're doing it for a "neighbor, friend, family member" (I've heard it all) then they would want their own rebate in their own name.
If you're getting multiples for someone who cannot shop for themselves, do your shopping with your rebates, then go back and do another trip with theirs after in a different check out. it's a little more hassle BUT less likely you'll have to explain the difference in names and info.
If there's anything else I can try to help with please ask and I can do my best