Pat and OP,
Excuse any typos, I'm on my phone...
My aftermarket temp gauge follows the oem gauge almost perfectly (or visa versa). Also, when I driver slowly or let it warm up just sitting, the oem temp gauge goes slowly up, following my other temp gauge.
Another thing I notice is the popping we get sometimes. When my car is very cold and you get on it (don't do that!) and shift, it doesn't pop. However, as the oem temp gauge goes up a little, in between the first and second hash, and you get on it, when you shift, I get an EXTREMELY loud pop, so loud it has scared people. Then, when the oem temp gauge reaches normal operating temp, no more extremely loud popping.
I say this because I based my driving on my oem temp gauge before I got my aftermarket, so that I didn't pop and scare people sheeetless.
Based on these things, I don't think the oem gauge (at least for the 05) is a dummy, but I do think that it works in intervals, maybe of 5 or10 degrees, and thus slowly rises, paralleling my other wt gauge.