I use plastic containers with snap on lids. They seem safer to me than jars since if the yeast builds up pressure they won't explode like jars do. I also split the slurry between 2 containers since an entire slurry is seldom necessary.
Between reading this thread and reading similar comments in JZ/CW's Yeast Book recently, I felt obliged to check on my yeast (in glass Mason Jars) yesterday.
There was some pressure build up in nearly all of them. The worst one was the jar that actually had the least amount of yeast/slurry in it (i.e., more headspace). Made a loud sound akin to a champagne cork popping when I opened it.
Kind of kicking myself for being stupid while doing it (I wasn't wearing gloves or safety glasses, nor did I have a thick towel over the jar in case of explosion). Not smart. No harm done, though.
I've left the lids on loose for now. Plan to eventually switch over to plastic containers and will use the Mason Jars for yeast rinsing or other short term uses.