As hell about that. While I don't beleive any of my reviews state that I allow prepay, I do. Which that, I've never asked for anything I've 50% of the booking fee, and never via iTunes. there are more than enough way for a girl to stay with the "cash is king" mantra, but still take some electronic payments. As Alexandra noted, I wouldn't expect a deposit be paid unless it requires travel (fore that's an mandatory overnight, or 3 hour booking if I'm within 3 hours of the intended outcall), or multi hour sessions.
Even in those cases, it's only limited to people I haven't see before in order to weed out time wasters that'll have you drive forever to never see you.
On the other side of all of that, I'd say roughly 1/4 of my current client base prepays appointments. It's a lot easier because of lack of ATM fees,along trips to the bank for large dollar amounts, a fair tee they end up on my limited schedule of appointments, and personal safety while traveling with large sums of cash. I also have one of my alternative income streams set up in such a way that my fee is then tax deductible as a marketing/advertising expense (invoices and all) for those that have play money built into their own companies.
Finally, I do have two clients that prepay an entire months worth of bookings when I'm back here in WI (they see me weekly) because it's just easier making one electronic transaction instead of 4-6 trips to the bank for stacks of cash.
I think a lot of it come down to the provider, as well as your own personal comfort level with it. If she's not willing to spend the time to pit your mind at ease with it, or if it gets overly complicated, just insist on cash. Drawback is that if she's working on first-come-first-served basis, you may end up with a stack of money and some blue balls.