Is it ok to spay a dog that was recently treated for heartworms?

We rescued a dog about three months ago. She was VERY heartworm positive and we treated her with the standard two injection protocol. We are scheduled to take her back for her follow up test in a couple of weeks. But she recently came into heat and is causing a MAJOR rift with our other dogs, one neutered male Bull Terrier and the other a neutered Jack Russell. Needless to say the JRT is not faring well in all this!!! I called our local animal shelter and they have a clinic tomorrow for low cost spaying and can get her in. Is it ok to treat a dog that *may* still have heartworms? The clinic doesn't have the facilities to go ahead and test her.