You may want to consider manifolds verses trunk and branch (if that's what's there). Also, the ID of pex is fairly small, so your main line may need to be larger than what you think. A 1/2" line should only really feed one thing. A 3/4" will flow about 2x the volume of a 1/2" line. You may want to run a dedicated hot water return line in case you want to add hot water circulation system later rather than using a cross-over valve and the cold water line as the return. Pex is floppy which means it is harder to make it look neat (but hidden in a wall, it really isn't a big deal!). For optimum operation, take advantage of the bendability of the stuff and avoid extra fittings to turn corners unless absolutely necessary. If you need a sharp corner, use a bend support. It looks more professional to use a copper stubout coming out of the wall for shutoffs for say sinks, toilets, etc. verses having the floppy pex there. They make copper stubouts designed to be anchored to the studs so your valve has something solid to hold it. Pex-A tubing is more flexible than the -B or -C variants, which means it can turn a smaller diameter corner without kinking. You don't want to make the holes too small, as when you try to thread the tubing through, you could end up scoring or binding and are more likely to crimp it. Type -A is the only one that can repair a kink without cutting it out and replacing it with a fitting. When using -A stuff, you can use expansion fittings which have less restrictions than those that use a crimp. If you decide to go with crimp fittings, make sure your tool is calibrated, or your test tool is the right one to verify you've crimped them properly.
Around the shower, if this is a tub/shower, you CANNOT use pex to the tub spout...it must be full-sized piping, and best with copper, or you'll have problems with the shower heads dribbling while filling the tub.