I would only increase the version number if users needed to download the plugin again. The "Tested up to" variable is not used when the plugin is installed, only when people want to install it or want to upgrade. In that case, the information comes from the server anyway, so you don't need to force a new download of your plugin.
Of course, if your
readme.txt in the
trunk directory has
Stable tag indicator, you should update the
readme.txt in the correct
tags subdirectory, otherwise it will get ignored. There is no problem updating a file in the
tags directory and not creating a new version, for Subversion it's a normal directory just like all others, it's only a convention to use it for tagged historical releases.