Sniper03 makes a good point. We assume most cheap lowers will work all right but no doubt there are some bad ones out there. As cheap as stripped lowers from known sources are, is it a good idea to buy unknown. Personally, I like to kick in a few more bucks in the lower to get something I'm proud of. Even $20-50 more is a drop in the bucket compared to the complete rifle yet it (the rollmark) really puts your signature on your build.
The biggest issue with assembling an AR isn't so much getting the parts together right - it's getting the right parts together.
US Army 1966-69, VFW Life Member, Retired Geek