My 1992 300D does the same thing (gets to the higher 90c temp range with AC on a long highway hill when it's hot out. Without AC it's just above 80c). Note that I have a new radiator, thermostat, headgasket, and water pump within the past 12 months or so. So my system is nearly as good as new. Mechanic also says it's normal.

But, I also installed one of these "cool harnesses". Works like a charm.

Link: K6JRF Auto Page

I have the 95 degree version and I believe it helps govern the temps well enough that I don't worry any more. Before that I don't recall the fans ever coming on even if my temp hit or bumped just above 100. I guess I rarely (if ever) hit 105 or 107 etc back then. But it comes on exactly on the 95c level on my gauge.