Only time I had an issues is when the saw is in the hot sun, then it pressurizes the tank. I think the way they do the vents now, seems like the let the air in, but don't always relieve the pressure.
In discussion of this problem at the conference, the tanks are not vented and will build pressure for various reasons. One fellow while fighting on a fire line, his saw wouldn't operate right and he thought was about to run out of gas. So him and his partner took a break, then later returned to the saw he popped the cap off to fill it thinking that was enough time to de-pressure the tank. He wasn't out of fuel but it had vapor locked from working among the fire and heat they were in. His tank was still pressurized and with much fuel still in it, it geysered onto him and with embers everywhere, he lite and received 3rd degree burns and spent over month in the hospital. Bad, bad, bad burns.
The FS now says to make sure your saw has cooled, open it with the cap away from yourself and have a rag over its top, to keep any fuel from being sprayed on yourself.