I too have had batches ferment out great without a single bubble. Although I will admit that looking back,I'm sure it was some fault of my process at the time in dealing with the yeast properly.
Anyway,I always sanitize my clean hydrometer & tube before any testing. The OG sample goes back into the fermenter,any other samples get taste tested. As long as your test tube & hydroemeter are clean & sanitized,returning the sample is safe if done quickly,imo. Never got an infection from doing it. Just use a little common sense. I also wait till 2-3 weeks now to start FG readings.