There are a fair few "good practice" things in yeast freezing you can probably sneak past if you want, but I figure they're simple enough that the added effort is minimal.
Pressure cooking pretty much guarantees no possibility of infection from encysted or spoored bacteria. Boiling is probably good enough, but I'd definitely go Angus' microwave method over boiling, once he tells you what it is. I'm not willing to risk probably, myself. You can pick up a pressure cooker for $10 or so from an op shop. I got mine for $12. Aldi sells a cheap one that lots of folks on this thread use.
You don't have to put the vials into a container with ice when freezing. However, the faster the yeast gets to freezing temperatures, the better the viability you should get. Pretty sure that was from the yeast book, but I may have seen it in a study. Can't remember. It's not uncommon for a vial to not actually freeze if it's ended up with an extra bit of glycerine. However, it's at freezing, and I haven't yet had problems with viability from one of these vials.
You don't have to use glass vials. You can if you can find ones with secure lids.