You'll need clearance for small amounts of deflection from having the handguard pushed against a rest, a bipod, etc. It won't need to be much. Unfortuantely the small handguards such as MI, Matrix, etc are all going to have minimal clearance with most low profile gas blocks. Luckily the industry has moved towards much smaller/lighter handguards. Back in 2005 the average handguard and as block probably weighed 30% more than they do now, but the handguards were huge so the need for a micro gas block wasn't a big deal.