I wouldn't do it. NFLX does seem euphoric and could be over-valued. But there's still probably a lot of people wishing that they were in it (don't you wish you bought yesterday before earnings). They'll pile on during any dip. So why short something that has demonstrated strength? Why not short something weak and in an downtrend like oil / energy related. I look at relative strength. When the market rallies today, I notice stocks that are not participating in the rally. Those would be good candidates to short when the market weakens. Trend is your friend.
I'm expecting a move similar to what happened with AMZN in april. A strong rise like today then a pull back and consolidation. I'm not looking to be in for long, maybe 2 weeks at most. We'll see who is right in 2 weeks time.