I haven't run any formal tests, but generally look for .002" under bullet diameter in smaller, lighter-recoiling cartridges, and .003" in larger ones.
Oh, and will also add that .004" or more tends to result in more misaligned bullets....
I recently loaded up some .30-06 rounds with 175 LRX bullet. Starline brass (this is good stuff) was LCD sized to provide .004" neck tension. All 8 rounds had .001 bullet runout. I look for .003 - .004" runout on my hunting ammo, depending on recoil level. My .260 Rem will get .003" and .35 Whelen gets .004". More than .004" in unnecessary IMO. Boat tail bullets don't seem to care much, but a flat base bullet is hard to seat straight with higher neck tension. A 250 Partition in the Whelen is tricky.
All my LCD mandrels are sanded down quite a bit smaller than factory provided so I can control neck tension by how hard I "squish" it against the mandrel. Work hardened brass and thicker necks provide higher neck tension.
I really don't think it's a huge factor in accuracy, especially in hunting rigs. When you look as the forces necessary to start the bullet moving, 50# is nothing compared to the force required to start the bullet moving forward. The start of gas pressure building in the case will cause the neck to expand before the bullet starts moving, making the bullet friction/tension with the case mouth even less significant. Just my $0.02
