There are 3498 teams in USA and only 64 of them can play in 2nd division (plus 16 in 1st division).
80/3498 = ~2%.
If we count 3rd league (256 teams) it's still less than 10%. It means that in over than 90% of cases potential = 5 is enough.
If You want to play in 2nd league then potential = 5 is of course too less. But only 2% of teams in USA can play in 2nd league. For other 98% of teams potential = 7 or better isn't needed.
Potential = 6 is enough for 2nd division (it's possible to build good team in 2nd division without better players than allstars). If You want to build unbalanced team (with few stars better than needed) then probably You need few players with potential = 7. If You want to build unbalanced team, or You want to prevent to unexpected cap because You want to train players higher than needed then maybe You should add one level to potentials written above. But it isn't necessary, it's only Your choise.