I've always changed gear in the upper end of the rev range, if I'm cruising at 40mph I always stay in 3rd gear. The engine seems to prefer it that way and I've had no problems with the car in 3 years (touch wood).
I've had my Doblo for almost 18 months now and I've recently changed from being a "change up as early as possible" driver to only changing up when I need to and I can confirm that the Doblo does seem a bit happier with this arrangement. The engine, running closer to the optimal rpm, also seems to maintain speeds better on longer runs meaning I seldom have to adjust the accelerator.
All in all, happy driving.