It is safer, cheaper & easier to spay them when they are out of heat as the area has less blood flow & is not as swollen. I worked in a vets office for years and I have seen MANY cats spayed while in heat (most were outside cats, barn cats, etc...) and they had no ill effects. Meko was in heat when she was spayed with no ill effects.

If you can not keep the cat safe from intact males then I would spay her now. Otherwise I'd call the shelter & ask if you could wait 1-3 weeks, if you explain the situation they should be more than willing to allow you the extra time.