Per usual, the question is, do you know you are free from STDs because you have both been tested? If not, then you do NOT know that.
The type of Herpes that is on the mouth (simplex-1) is not the same type as gential Herpes (simplex-2), but either type can infect either area.
That given, you should indeed be using a condom when you even suspect you MIGHT be getting a sore. Transmission can only occur when a sore is active, but that time period starts when the sore is first developping, before it is obviously there, and ends when it is COMPLETELY healed. If you are aware of these cycles well enough to feel you can accurately tell when they are, a condom is not absolutely neccessary, but personally, I'd advise it.
Why take that sort of a risk? Condoms are easy to use, and Herpes isn't the only thing out there to worry about.