I think expansion until 100 was recommended because of lack of flower gems at the start and because you won't really be getting 5* manyu to hoard for leveling. but once you start farming the 80 stamina dailys (dragons) and chapter 12 maps for both ampy and 80 sta SG, you should probably expand as and when necessary. by the time you get to this point, you probably would've already spent 50 or 100 flower gems on rolling at least (which was all that was necessary for a stronger start), so my guess is start expanding any time after your 1st or 2nd 50 gem roll.