Mine is usually loosend all the way, with enough to grab for when I go on air. If not on air, it's either done up loosely under my chin, or over the brim. Depends on what I'm doing.
I keep it loose under my chin as does the author of this post.
What I have found is it's much easier to just tighten the strap when needed. What I have found is the guys who like to wear the strap up over the brim, usually forget about the strap when thrusted into action and they are also the ones who are reaching/searching/looking for their helmet when it is rolling around on the floor.
I then make a point to tell them they look really cool when their helmet falls off at an auto accident, or in a fire.
Straps should be worn on every call. If your wearing your helmet (as you should), then the strap is a part of that and needs to be worn. NO IF ANDS OR BUTS! There is no good reason not to wear it. Don't wait for the unexpected to happen and then say " guess I should have worn that". If you can walk by and bump someones helmet with your hand and it comes off.....they are not wearing it properly.