I'm not sure that's actually true. Years ago at Camellia Forest they had a bunch of named C. deodara liners, and they sure didn't look like grafts. If they were, it was done when they were incredibly tiny under very controlled conditions. No signs of a graft union. I think the commercial liner producers might have cracked the code of rooting them.
That being said they are probably not the easiest to root. Maybe someone else here has experience. After failing with some rhodie cuttings last winter, I'm trying a back to basics approach with easier things: holly and camellia mostly. I am trying a couple rhododendrons again. This time I'm using bottom heat in my garage window instead of having them in my house. I'm told the roots being warm and the tops being slightly cool is the recipe for success. Funny you mention this actually, because I was just about to pull the trigger on a $30 heat mat thermometer, but decided to just measure the temp. and use a 24h/48 position timer I have to try to get the temp. stabilized around 70F. My mat is about 10-12 years old and I cannot remember if it has a thermometer or not. It seems to keep the soil temp around 75F, which is probably a little too warm.

Well you pushed me over the edge because you reminded me to go out there and check the temperature. My attempt to run it 50/50 (30 min on, 30 min off) had caused the soil temp to drop to 53F! It's going to be hard to keep a stable temp. w/o a thermometer. Also, on warm days in winter (which don't really happen around here compared to DC or even Baltimore) it could get too warm even if I do find a pattern of on/off that gets close to 70F.
Got this one.

Here is a link that might be useful: http://www.amazon.com/Hydrofarm-MTPRTC-Digital-Thermostat-Heat/dp/B000NZZG3S