well I actually prefer my helper squads at 600~700, but I often have to settle for 500-600. anything below is pretty rare unless I have no other high power helpers =/
my own helper squad is at 400~500 though, for max power

My teams are all around 400-500 save for one team in 550-range, since I still use a fair amount of grass/bronze (the ones that are cuter than the silvers :P). My strongest golds are on the slow side too (Cosmos, Lily Rubellum, Lavender).