CAT5 or 6 to projector:
1. You need one cable for either IR or RS232 to control the projector;
2. More and more projectors have ethernet connectivity for control, remote diagnosis, set up, proactive notification of faults or predictive failure, so you need a Cat5/6 for that (some systems provide IP for set up and diagnosis but won't provide the ability to use the IP connection for interface to a control or IR system);
3. Should you have an issue with your HDMI cable, now or in the future, you can use a balun or HDBaseT device to run HDMI over one, or two, CAT5/6 cables.
That then is a total of four CAT5/6 cables.
When would you run five RCA cables? You may have legacy devices in your system which will not have an HDMI output ... so unless you want to replace those items, you need to provide for RGB or RGBHV input into the projector.