Definitely sound first! I have a theory that all guitar buyers should be blindfolded!

But here in the UK shops will usually refuse to put new strings on. Even had that when (with a friend) buying a Gibson J45 - about GBP 1000 - and the shop refused to get more than a couple in from the warehouse (its own warehouse, stock it owned) to compare, and point blank refused to put new strings on unless we guaranteed we were going to buy or pay for the strings.

In short, they couldn't have given a stuff wheher we spent GBP 1000 or not!