Doesn’t phase me much. I’ve been venting the middle D all along, so it’s just reached a point where that’s how my fingers move. I’d be in a lot worse shape if I had a situation where I really needed to NOT vent it.
It all depends on the siuation. I find that venting helps to jump the octave cleanly. But by not venting you can seriously overblow the middle D giving a wonderful, forceful note which can punctuate phrases really well (you wouldn’t guess that Conal O Grada is one of my favourites would you?). You can also get a cran to really pop out of the flute if you don’t vent. As others mentioned, if it’s a quick passage or an awkward passage to finger, 
then why bother venting.
Graham
good points, helpful. Thanks, Graham