It depends on where you live. For example, I took the subject test last april at CIC-IPN (Mexico City) because I live about 2 hours' drive from there. It's comfortable (the classroom is big and quiet, and we were only 3 persons when I took the test). I only recommend you to set aside your place as soon as possible, since it is in high-demand in October and November. I know a guy who had to travel all the way to Mérida because he didn't find any places available in Mexico City for the test of November last year, even though he lives there (Mexico City), so just plan ahead.
btw, sorry about my english, it's not good yet