According to the Plano Municipal Court site in order to be eligible to take a Driving Safety Course to dismiss a citation a Defendant must meet the following six (6) requirements:

1. The Defendant must have a valid Texas Driver's License.
2. The Defendant must have proof of financial responsibility (insurance).
3. The Defendant cannot have completed a Driving Safety Course within 12 months preceding the date of the alleged offense for the dismissal of another citation that is not yet reflected on his or her driving record.
4. The Defendant cannot have been charged with speeding 25 miles or more over the speed limit.
5. The Defendant must enter a plea of "guilty" or "no contest"; and
6. The Defendant must pay the State Costs and an Administrative Fee, totaling $95.00, at the time of his or her request.

Number 3 seems to imply that you can only take the driving safety course once every 12 months. You may want to check with the Texas Department of Safety to find out if there is a limit to the total number of times you can take the driving course in your lifetime.