Yes.  I went camping 2 weeks ago and we went to bed with them open.  We closed them partway overnight as it got colder. 

As for feeling safe.....it's canvas.  Zipped up or down, if someone wanted in they were going to get in. 

However I will say I replaced the screen door handle with one that is keyed because it makes my wife feel safer  ::)  I started to explain how the canvas could be pulled out of the door track or unhooked from the bunk ends but then I just shut up  :-X and let my wife win that argument.  It cost an extra $3 for the keyed handle -vs- the non keyed handle.

ours are always zipped open partially.  helps keep the condensation down inside the PUP/Canvas...

never slept with them wide open though

Without the windows open somewhat, I would wake up with condensation dripping on me. And our dog loves to look out into the darkness.

Love to sleep with them wide open...but here in Florida, that isn't very often...Like January! LOL.. Seriously, when the weather cooperates, the windows are open.  Just a side note, last January we camped it got down to about 28 degrees and we were in Central Florida, not northern Florida.

with all three of the pups we've had, we almost always kept the windows open a little bit. as already said, helped keep the condensation down. we have slept with them open all the way in spring or fall

It depends on the circumstances and if the weather and wife cooperates  ;). I like them about half way open. this allows me to see out at night but half way restricts the full view into the "mobile canvas vacation home"

^ That's different. We always leave at least one wall halfway up at home!  😀

Open 'em all if the weather is right.