When writing I saw in French do you add an e to the verb if the subject is female?

The conjugation doesn't change in French when the subject is female rather than male. Ex: Robert chante, Julie chante. That makes no difference whether is is Robert or Julie singing. If the verb doesn't have in its normal conjugation a final "e", you don't add one.

However in any composé tense where the verb uses être instead of avoir, there is an extra "e" added for the subject, but only in this context. Ex. Robert est sorti, Julie est sortie. If in the composé tense, the verb uses avoir (which is dominant), there is an extra "e" added for the direct object as long as it precedes the verb - not the subject. Ex. Le steak-frites que Robert a mangé... vs. La dinde que Robert a mangée... BUT Robert a mangé de steak-frites -and- Robert a mangé de dinde.

In the case presented in the question, I saw, in typical passé composé would be "J'ai vu". Since this is an "avoir" passé composé, changing the gender of "Je" does nothing. It will only take an additional "e" if the direct object is female and precedes the verb, e.g. I saw her = Je l'ai vue.