It's second nature to put the weapon selector to fire when preparing to shoot...because it's on safe otherwise.
Once you've built putting the selector into fire (semi first typically) as part of the movements of bringing the weapon up to aim...it becomes so quick it's not something thought of consciously. Having your thumb resting close (or on) the selector lever as it's in safe makes it really convenient to flick it to semi as you close your strong hand grip for aiming/firing.
On the other end of the firing sequence, if you practice putting your rifle into safe everytime your bring it down out of aim/firing position...then you never f*ck up your aiming sequence.
Doing so...you never should have to guess as to what satus your rifle is in (i.e. fire? or safe?)...you know that when it's not shouldered it's safe...when you're ready to fire it's on fire. You don't have to look, glance, peek at it. You don't have to worry/sweat that your rifle is going to go off as you creep/crawl through the brush.
And as the best side product of doing it this way...the rifle is on safe (not the safest condition, but at least safer than ready-to-fire) when you're moving/hanging with your teammates. Shooting them on purpose when it's called for is one thing
...shooting them accidentally just makes you look like an idiot.
I'm more of an armalite guy...so this is pretty natural for M4's and similar with my FAL. I guess that it'd be different to do with an AK since the selector typically has to be manipulated with the off hand and requires contorsions or turning the rifle over.