Hi Guys,
6 ml safety glass is a personal preference, not a minimum requirement.
As my partner has a couple of kids who visit on weekends and run riot around the flat, the saftey glass gives me extra piece of mind for the snakes' (oh and I guess the brats') safety should there be any breakages. I'm sure there are plenty of people who can advise you better on minimum requirements, but I'm very happy with the safety glass.
Just a note, if you go this option be prepared to pay more for it...
Cheers,
Nic