In general, I would rather have someone ask if I need assistance than not ask. It's common courtesy as a staff member to try and be the most helpful that you can. A good way of doing that is knowing what they're looking for and that comes about by asking.

There is nothing helpful in  distracting potential buyers from having a look around.  Unless I know what they actually have, and if I like like it, I do not really know what I want; so how can anybody help me?  Sales staff should only ask if person need assistance if they look as they need it.  I am talking about clothing. 

It may be different if I walk into a store looking for something particular, like a fridge, but even then, I find sales staff more interested in selling you something, anything, rather than actually listening what you are after.  I have had a girl trying to sell me microwave when I wanted ordinary portable oven, or when I was after very specific upside down fridge they tried to sell me something 2x the size, not to mention that when I asked for stainless steel kettle I ended up with one that is plastic inside.  If I was inspecting the kettle myself, I would have looked inside.  Not to mention the man who dragged me away from  digital radios to try to sell me cheap clock radio.  He actually said:  "they are digital, they are very expensive",  I can tell you I was out of there fast 🙂