I feel that for most people a half centimeter jump in stem reach is not necessary. The reason that small jumps in seat height are so easily felt is that the body has no way to compensate for these changes without either lifting the heel at the bottom of the stroke or grinding the pubic area into the saddle or even worse - rocking on the saddle throwing off cadence and efficiency. The difference between a 10 cm stem and a 10.5 stem is barely felt if at all by me. I can make up the difference with a tiny change in my elbow angle. (We do all keep a small bend in the elbows all the time - don't we?) If you raise the stem even a smidge, you also shorten the reach by almost the same amount. The difference between the reach of two bars from the classic period is easily 1 cm or more. If you bring your hands to the hoods as opposed to the drops the difference in reach is quite a few centimeters. If you put the brake levers further up the bar curve you effectively change the reach also when on the hoods. If you use a 172.5 crank instead of a 170 crank, you lengthen the stem reach an effective quarter of a centimeter. A thicker bar tape shortens the reach by a quarter also. Changing the angle that you secure the bars to the stem changes reach too. If you use bars that are smaller than what is called for - for a given stem clamp area, you can lengthen or shorten the reach depending on how and where you apply the shim. If you ride in the morning when your spine is as much as a half inch longer - do you switch out to a longer stem? I don't
Garth Libre who uses 9's 10's and 10.5 stem interchangeably. (in Miami Fl USA)