It depends on the severity of the astigmatism. If the astigmatism is greater than +/- 3.00 diopters then it would be a disqualification. A waiver would depend on how much over the limit the applicant was and whether or not there were other vision issues.
If you feel that your astigmatism is eye trouble then mark it as such. As I've stated before I'm not going to answer the medical history questionnaire for people. If you do check the yes on eye trouble, DoDMERB will take a look at your answer for the eye trouble block. If they feel that it is not an issue they will just ignore it.