No doubt about it, time spent when it comes to home or even cottage plumbing taking the time and effort to plan and measure rough plumbing is overall a good investment in your time, sanity and overall plumbing or heating renovation costs and construction reno budget. It's more than worth your time and effort when it comes to plumbing or heating pipes to plan piping layouts ahead of time and project.
Overall professional plumbers and contractors will state as a basic rule of the trade and of thumb that it is best to take line of sight tact and approaches - that is to take the most direct route and avoid obstacles wherever possible. It's as if you take a direction that the crow would fly - even though it's in your house, cottage or other form of real estate property.
The supply pipes can usually be routed parallel to drywall but without slope. Make it a general rule that wherever and whenever possible to follow the adage to install existing fixtures back to back tied into the "soil stack" arrangement system. If you have a home that is two stories or more you can work out these plumbing fixtures "piggyback" style with the new fixtures on the floor below or directly above the current existing setups. However think about it and add the thought to your mental thought patterns that you might just have to totally separate vent pipes to the lower plumbing or heating fixtures.
Many homeowner's have discovered with renovating experience and working along with established contractors and plumbing trades people that often a new tub, sink or alternatively shower can even be tied in directly to what is often referred to as a pre-existing "branch line". All in all you're well known local "Mike the Plumber" that this is the tried and true method, the simplest of all and most cost effective means of accomplishing your purpose. It's tried and true and on top of that seems to have the least overall problems and concerns over time. This layout is more than time tested with the contracting and renovation pros. One point to consider and take into account is that your local state building codes may require a toilet to have its own drainage pipe or alternatively to drain fully into a branch line that is at least 3 inches in diameter. Its code.
Lastly when it comes to the trap consider that the distance between the trap of a fixture and its vent pipe is also given strict considerations in code that you must adhere to. Check and verify your local code building standards. A simple phone call may save you a lot of wasted time effort and redoubling or renovations and costs that all in all could have been simply prevented thorough simple preparation, layout, a chalking of the lines, asking questions all in advance of final renovation tasks and construction.
Remember all in all , taking the time and effort to plan pipe and piping layouts , to do a proper and full measurement of rough plumbing is more than best economy overall. Remember the adage measure twice and cut once.