It does no harm to the fork seals. It doesn't build pressure like that.I use 2 straps per bike a piece of heater hose round the bars where the strap goes, and only pull down enough to keep the bikes where I want them, I've seen some guys almost bottom out their forks tying the bikes down, I try too save my fork seals for the ride.
I can take the memory out of the springs , but if i'm going to winch mine down, Its only for the ride home, IF i think its going to be any longer i throw in a brace.