Head piercings will be bloody, whether the penis is erect or not. Moreso when erect, but it is for sure going to get hard if not in the studio, then later on.
Piercing while hard vs. soft is a preference that might vary from piercer to piercer. Some piercers and clients/patients just want to get it over with, but only with an erect penis can jewelry be correctly sized. Then there are other male genital piercings -- such as a frenum or frenum ladder -- that really should be marked, if not pierced, when erect, else the risk of the alignment and positioning changing.