The question
If a waiter offers me a sample pour of a wine to judge whether I like it, is it acceptable to finish the whole pour after I've agreed to order a full glass?
The answer
Sort of, but you should consider doing it the classy way, by asking the waiter, "Mind if I do?"
I confess to having had an awkward moment over just such a situation at a restaurant in California not long ago. I knew the waiter, Hector, pretty well because I was a regular. On this occasion he provided me with a free splash of a wine I was considering for my meal. (He was great with free splashes, which, now that I think of it, may have been part of the reason I became a regular.) He eventually left the table to serve other patrons. Upon his return, he saw my glass empty and said, with deadpan delivery, "Hey, you're not supposed to finish it." It was a joke, and we laughed -- once I got over my momentary embarrassment. I normally don't finish the whole pour, so I was either particularly thirsty or more absent-minded than usual that evening.
But Hector was getting at a truth, or at least what some in the industry consider a restaurant-etiquette rule. If the first sip is sufficient to pass judgement on the wine, it's considered gauche to throw back the whole sample as though you were a frat boy presented with a tequila shot. The waiter sometimes will ask if you'd like to keep the glass while he or she brings a fresh one. More commonly, though, the fresh wine will be poured on top of what's left in your glass to bring the volume up to where it should be for a full serving.
I think it's a good idea to ask if you can finish the pour before you do so – even if the waiter is a good-humoured friend.