My rules for tertiary roads are:

highway=tertiary {set mkgmap:no_name=yes} [0x1101d resolution 18-18 continue]
highway=tertiary {set mkgmap:no_name=yes} [0x1101b resolution 20-20 continue]
highway=tertiary {set mkgmap:no_name=yes} [0x1101b resolution 21-21 continue]
highway=tertiary {set mkgmap:no_name=yes} [0x05 resolution 22-22 continue]
highway=tertiary {name '${name}'} [0x05 road_class=1 road_speed=3 resolution 23]

(if mkgmap:no_name is set, <finalize> scripts are not executed, so  name for road is not assigned).

As you may suppose, tertiary roads should be seen on almost any zoom level.

MapSource on 300m (level 23) zoom:

http://imgur.com/LHptaRJ

MapSource on 500m (level 22) zoom:

http://imgur.com/1Aa4SL9

Part of the road disappears......

This is not related to any possible MapSource or Garmin (tested on hardware) bug with road rendering. I opened IMG file in MapEdit and it really this lacks road on levels 18-22.

I found that this bug is related to --preserve-element-order parameter. When it's set - everything is fine. Without it, road disappears.

best regards

Michal Rogala