It depends on the store, but usually they ask me if I need anything right away, and if I hesitate before saying "no" they push to offer any help. That's usually what I need anyway, but I'm always reluctant to be like "I'm here looking for this one specific item, where is it?" and would rather browse. But since they pick up on that hesitation they know that I really *do* need help with something, I'm just not saying so.
That said, the store at my local suburban mall is much less clingy than the stores downtown in the city, and I prefer shopping there. If you say you're browsing they'll let you browse without hovering, even if you're the only one in there (which is often the case). As soon as you look up, though, someone will scurry over to offer any assistance. I actually really enjoy getting help at that store, since they're more than happy to go digging around looking for exactly what I want, and they always encourage me to try out everything before making any decisions. It's made the difference between leaving empty handed and making a purchase I'm really happy with.