As with most parts of any remodeling project, planning is critical. Of course you still may get a better "sense of the space" after the demolition is complete and change things anyway. I still recommend being ahead of the game.
My first construction job was on a hospital. One of my tasks was to install wood bracing in the walls for grab bars in the bathrooms. There was to be something solid to attach to for these applications. In the typical residential bathroom, you probably just do not want your kids pulling the towel bar off the wall. This little detail never escaped my memory banks.
When you are through with the rough in work on your bath project, and before you hang the drywall, place your towel bars, grab bars, paper holders, etc. Document these locations (believe me you may not remember). Then install framing in the walls for a solid connection for your hardware. We usually use 2 x 10's for a wider location option. We also put more than necessary for the same reason. Of course, in the end, you may once again change your mind and have to use sinkers in the drywall anyway. If not, the effort will be well worth it.
The ultimate solution would be to install plywood on your walls under the drywall. Then you have complete freedom with your desired locations. Though, this will shrink your room slightly, and cost a little more. The plywood is also a good option when building a handicap shower. We then use pressure treated plywood under our tile board and mastic adhesive along with the screws for an added moisture barrier.
Planning your bathroom remodeling with these tips in mind will keep your towel and grab bars safe and secure. If you are hiring a remodeling contractor for your bathroom request that the extra support is installed. Taking these extra measures assures your hardware stays in place for many years of use and enjoyment.