I am currently running conduit for Cat5e cable in the home office I am finishing off. Are there any rules in the code for how far off the floor Category 5 wires in conduit must be run?
No. Cat 5 cabling can be run on the ground or underground. If you are worried about it working just make sure the coating isn't torn/cut and it is capped properly. If you are worried about code violations - no inspector cares unless the cable is interfering with something else he cares about.
My experience level - ran communications cable for large internet provider for 4 years both in the field and in-house. If you run cable underground there is code specific sheathing needed. Not really sure this would apply for cat5 because that wouldn't be used for long runs. I personally have buried cat5 cable 1 foot under by 50 feet to set up a link in a shed that now works just fine 8 years later - without piping. Hillbilly but works perfect.
If I were doing my house I would run it 7-8 inches off the ground. Just because that is usually a spot that has little chance to be nailed/screwed.
Actually I wouldn't run cabling in my house. You run cables now and in 2-3 years wireless is faster. Seriously what could you be doing at your house that wireless wouldn't work? They make 1GB wireless routers for under $100. Soon 10GB will be the norm. Your internet speed - if it is blazing blazing fast - goes at 100 MB. That is 10 times less than your 1GB wireless throughput. Also your computer will only be able to process so much data being saved to it. I have my house teched out to the max (video/music hub server, print server, 10 devices online, virtual servers, mac/pc) and I haven't thought for a second running cables. Unless you are looking at selling your house soon to a couple in their 40/50s (the only demographic where cabling was known to) I see this as being pointless from a pure tech standpoint. My advice - take all the money that you were spending on this project - read reviews - buy a good router, buy a repeater if you are worrying about reception to devices, buy a central hub device (NAS) to store and share your stuff and enjoy.
IMO in a commercial setting all receptacles, switches, jacks & the like should be installed at what is called counter height (about 48" to the top of the box). This not only satisfies ADA laws, but it keeps it all above most furniture, in sight for quick inventory, easy access, easy repair and upgrades. The days of crawling under desks and counters to get to plugs, kicking jumbles of cords plugged into recepticle strips under desks or smashing furniture against plugged in cords and cables really should come to an end. Exceptions to this would be common areas, corridors and lobbys