-Depends on how you approach it; if I am just there to gawk, I tend to let the staff know immediately - leaving them to decide whether they want to spend time chatting with me if it is a slow day in the store or just leave me to my own devices.
Incidentally, best service I ever received was when oohing and aaahing my way through the Wempe on Kurfürstendamm in Berlin a few years ago; I let them know that I was just there for looking at their A. Lange range, possibly looking to buy in several years' time, if times were still good in my business - and was promptly showered with (polite and welcome) attention, given a tour, tried on lovely watches until I had to ask them to stop putting them on the desk in front of me, being sent off with a tote bag full of catalogs and a couple of books.
Result? When and if I finally feel comfortable spending such an insane amount of money on a watch (I've almost saved up enough, over a period of more than seven years, but with current job security similar to that of someone hawking The Satanic Verses on a Teheran streetcorner, shopping luxuries is Out with a capital O) - I'll be going to that very same store, and I'll let them know precisely why I've come all the way from just under the arctic circle to pay them a visit. Because of their stellar service ages ago.
(At some point I really, really want to own a gold watch; hopefully either a A. Lange Saxonia (thin) or a Patek Calatrava 5196R - IMHO the Lange hands are more esthetically pleasing than the Patek ones, whereas the Patek has the upper hand with regards to the case, in particular how the lugs are attached to it...)