If your PS is before the MK opens I would take the monorail to be safe. The resort monorail starts running at 7:00 although I just learned from one of ChipnDale's posts that they don't drop off at MK until 7:30.
I don't know what time the launches officially start but I do remember waiting for one last summer around 8:30 or so. We waited quite a while (10 minutes??) and didn't see one anywhere. We finally gave up and started to hoof it to the monorail hoping to beat the crowds that we could see leaving 8:00 mass. We waited a while for the monorail and then as we were approaching the MK we could see a launch nearing GF. Maybe they start at 9:00? I hope someone knows for sure.