Oddly enough I saw a lot of girls whowere working only for 1 month and were of extremely high quality during October of last year and September of 2 years ago.

The fall, from my experience.

The only time NOT to go to TJ if you can help it is during the Christmas Holidays, from late November to some time in January.
Many of the hot girls have made their money and are home for the holidays and slowly come back to TJ.

If I had to pick a best time, I'd say it would be right before the Holidays, around early November. Lots of girls are out to make as much money as then can so they can take off the weeks or months for the trip home.
The weather in TJ can be a bit cold and sometimes it does rain and the streets are miserable.