I agree with above- I suggest talking about what she and you want before you get to that moment. It doesn't have to be a huge heavy discussion, but some women like it when men ejaculate in their mouths, some don't, and some are up for trying it. One thing- if you cum in her mouth, be prepared that she might gag on it. If this happens, please be a supportive partner and let her know it is okay, and that you are still into her etc.. The first time my bf came in my mouth, (one of the first times I'd ever given head) I gagged and I was horribly embarrassed. Fortunately he gave me a hug, asked if I was okay, and we continued our fun with him continuing to encourage me. I have since almost learned to control my gag reflex, If your partner isn't used to it, it might be really scary and embarrassing and a supportive partner is the best way to keep fellatio fun!