The key is how often they will be useful.
Though 5 heavies + 1 Sniper certainly sounds like the turtle strategy.
5 properly built Colonel Heavies = 18 rockets = more rockets than aliens in most abduction missions. Before then you're still easily looking at 10 rockets and 5 bulletstorm heavies. Hardly a strategy that needs turtling or, well, strategy.
Anyway, I take whatever the RNG gives me in regards to squad composition. Experience is more important than being the right class, and when you reach all max rank it basically doesn't matter what you do especially since you'll have ghost armor or plasma or something else game-breaking at that point.
Barring large amounts of death, my guidelines are: one of each class, and either a second support or the heavy gets a medkit, then the remaining 2 slots are filled by anything that isn't snipers. Snipers are either largely useless (squaddie rank, out of position in urban terrain, or meeting aliens right at mission start) or WAY too good (any other time), so I don't find them fun except in small doses. One squadsight is a must though, for the damage and utility. One assault with lightning reflexes makes overwatch so much easier to deal with. Heavy's rockets are a get out of jail free card if things go bad, and supports smokes are similar if worse. Support medics let you heal without worrying about not being able to stabilize. So yeah, I basically want every class on my team.