If it can soak it up in a few hours it's fine to leave it. If it stays any longer then I'd remove it.
So it depends on the size of your plants and root system. If they are growing well, you'll have no problems with a bit of standing water. It's hard to over water using promix once the plant is estblished and your not over doing it.
*I* let it sit for 1/2 to 1 hr and then dump the saucer if there is any left. Sitting in water is not good, but a short time period for the mix to absorb from the bottom isn't going to hurt anything.
You should look into "bottom feeding" with those bags, works great. just harvested two that were bottom fed the last 6 weeks or so and the entire 7gals was one beautiful root ball.