Trying to remember where the pluming is, and then cutting a hole to get access tends to be a messy job, and that's if you get the location right the first try. More often than not the first try is a miss and another hole needs to be cut.
With a better safe than sorry perspective, I installed hatches at each location under the bathroom where there were key plumbing fixtures. The hatches are held in place using a spring-loaded lever. All that you do is cut the hole about an inch smaller than the hatch cover, insert one end of the hatch and hold the spring lever while you "lower" the other side of the hatch over the hole.
The spring keeps the the hatch cover firmly in place even in a ceiling. These hatches work great in ceilings and walls and come in various sizes to fit the size of hole you need to cover. While not invisible the hatches have a nice smooth finish and hence are not at all offensive.
Now I hope I never need to open the hatch!
Search Amazon.com for Access Hatches