Where the heck did you hear that you have to keep your left hand wrist straight. It's bending and changing positions all the time depending on what chords and leads you're playing.

Just keep in mind where guitar is concerned that there are no 'rules'.
Just because somebody else (usually a teacher with preconceived notions of what's 'best' for beginner guitarists) says you have to do it a certain way doesn't mean you have to follow that advice religiously.

Hell... a lot of my students tell me their teachers tell them they aren't supposed to use their left hand thumb and to keep it anchored behind the neck. My advice is that's the dumbest thing I ever heard of. There's a whole bunch of chords you can play that can only be done with your left hand thumb holding the bottom E string or muting unplayed strings.

My advice is do whatever feels natural and helps you achieve what you want to do. Over time you'll find the best positions that fit your particular playing abilities. (not everybody has the same hand size and style)