Well it's a different hole, so you won't wash it out! You will however relax your pelvic floor muscles to pee and that might mean that the sperm is released at the same time. Apparently the good swimmers will be up through the cervix in just a few minutes though, so anything that comes out won't be worth keeping anyway!
I've had many UTIs since TTC. Definitely get up and wee to avoid them. euro is right, the good ones are heading in the right direction already. Any that you have to encourage to stay aren't ones you'd want anyway! I got pregnant despite going straight off to pee afterwards.