Hi Ceci,
I don't think there is anything wrong with a quick text to say thanks and you had a lovely time. As long as you have got a handle on the situation, as in, he is driving from your place and you know he is, rather than sat at home drinking his cocoa!! It's simply about the timing.
These guys are not stupid, odds are they will not be calling you on their regular mobile anyhow, I have seen a couple of guys with 2 mobiles! Just don't leave it half an hour after you leave or they leave.
You can always tell them you had a nice time right as your leaving you know. If I have seen a particularly nice man, I have made a point of thanking him for being lovely as I am leaving or he is leaving. I know it's corny, but if you catch the right tone when thanking them and adding something like, seriously, I mean it, you made my job a pleasure this evening - It's like praising a child, the smile that crosses their faces, honestly!
I also get texts or emails afterwards thanking me and I always make a suitable reply to them as they come in, I don't wait unless I am on a booking or otherwise can not get to reply at that point.
The point was made on here not too long ago that we are paid to go away and that is true and has stuck with me as a piece of sound advice - however, there is nothing the matter with good manners and of course you want to try and cultivate regulars and a good reputation.
Just use your instinct and common sense and mostly, thank them as your leaving them. It's worth the extra 5 mins at the front door...
Good luck.
Jo