Shirt tail tucked in or not?

No you do not have to tuck shirts with square bottoms and side vents, only shirts with tails. My personal opinion is that men look geeky if they tuck square bottom shirts in...the pencil protector mentality.

Business casual in general would include tucking in the shirt in all cases. Maybe on a casual jeans Friday you could get away with not tucking in your shirt in a relaxed office. Sometimes it depends on the company and the atmosphere they set, but more than likely the usual standard applies. Sometimes an office with an overwhelmingly young crowd (80% between 20-30/etc) I've seen more relaxed attire.

I have no clue what side vents are though. You got me on that one.

If you want to appear to have style tuck it in. If you want women to think your a slob, just wear it out and don't iron it. They wil not give you the time of day.

It's up to you, which ever you are comfortable with. Really! Just be sure the shirt and pants are nicely ironed.

you can go either, use your judgement. If you do tuck it in then wear a nice belt

tucked in.otherwise it looks like your hiding flab.