It depends on the type of convention you go to. For the most part, cons with a lot of cosplayers in attendance, people will know it's general etiquette to ask before touching. Even if I really want to feel something to see what it's made of, I always ask the owner first. When I wore my Iron Man suit, people just asked what it was made of, instead of trying to find out for themselves by touching.
Sometimes it's unavoidable though. At really packed, busy conventions, it's difficult to get around without a handler. You need to judge for yourself if you need a handler, or if you just shouldn't wear that armor, or carry that prop inside and remain in an outdoor space instead where there's more room to move around. Store breakable props in the car or hotel when you've decided to go inside for panels or shopping. It's all about planning your day. If your costume is too fragile to wear all day, then don't wear it all day.