I've lived in a 40+ year old house with a slab foundation, and there were no problems with cleanout, etc. Think about how often you really re-plumb a house... On a new construction, you should consider the material to be used as well as the number and locations of cleanouts and floor drains. I would use 4" waste lines and at least 3/4" supply lines for all buried pipes. Other things to consider are the soil in the area you're thinking about (for drainage) and the water-table level. Are there other, older houses in your area with slab-foundations? Talk with your builder and let him/her know your concerns. Best of luck in your construction efforts!