Hi Kaz - Welcome to BB!

My hospital recomends you commence classes between 30 and 35 weeks. I had to book in when I booked into my hospital (a private hospital) and even though I put my request in straight away (before I even reached 8 weeks) I missed out on the classes DH and I wanted and ended up going when I was about 25 weeks (from memory). Personally, I think that the earlier you go the better - there were some preperation things mentioned in my classes that would of been little use to those women who were only 10 weeks (or less) from their EDD!
I had to book into my classes through my hospital. My Ob's receptionist booked me into the hospital who sent me out registration info including papers to enrol in my classes. I sent it back to them and got in. There are some private classes around that you can enrol in or contact the hospital you're birthing at to get details of their classes. Some Community Health Centres also run these classes (or can point you in the right direction). There is a thread around here somewhere about local classes but I haven't looked at it in a while so am not sure if it's only Vic classes or classes around Aust - but have a look at it!
With regards to being more prone to tearing if you have an epidural - I haven't heard that one! Maybe one of the other girls might know more about that though...

Hope the rest of your pregnancy is a H&H one!

MG