I'm going to disagree--somewhat.
I agree it's best to bring dollars. When you do bring lots of one's and five's as nobody in TJ has change.
Most ATMs only provide pesos, but if you want pesos you'll get a better rate using an ATM than you will at a casa de cambio. Personally I carry both pesos and dollars. In most of the places mongers frequent you're better off using dollars, but there are exceptions (plus I live in TJ) so I find it best to carry both.
Caution needs to be exercised when using an ATM. Don't every let anybody handle your ATM card, don't let anybody see your PIN, etc. Try to use a ATM connected to a bank and above all look out for rateros.
I use my ATM card frequently, but I prefer using an ATM that isn't in the Zona or places rateros stake out. I've never had any problems using an ATM in Tijuana. (Except the time I forgot and left my card in the ATM. Somebody turned it into the bank, which returned it to me. And, as I said, I use an ATM connected to a bank that isn't located where "low lives" hang out.)