Is it a must for Hermits to HP-wash?

it's not necessary, but it can be extremely helpful. it allows for bossing and training in certain places at lower levels, helps you survive without hb, etc. i personally wouldnt sacrifice the damage for hp if you don't wash, i'd just leave it alone. eventually with hp equips you will be able to survive all bosses currently released in GMS, (no clue about EMS, and upcoming bosses deal more damage) but reaching those levels is tedious and most people would end up quitting before then.

basically, if you have the money to spare, wash. if not, don't worry about it.