I wasn't a TEFL volunteer, but from the group I was in, the level of experience TEFLs had definitely varied. Many had teaching certificates, some did not. Some had TEFL certification but most did not. I think TEFL is definitely one of the more 'competitive' fields (I hate using that term in regards to pc placement) but there was extensive training for TEFLs when they arrived in country. I am not sure, but I believe they may have gotten a TEFL cert during their training. They definitely got plenty of hours of practice teaching and making lesson plans.
I would definitely say that any tutoring or teaching experience will help you in your service. Also, if you can somehow show that you are able to create lesson plans and work on your own.
A final thing to keep in mind: Even if you are not officially TEFL, almost every volunteer ends up teaching or tutoring english at some point. So, brushing up on those skills wont go to waste.