Every wordpress template or plugin author writes in the header of the theme or the plugin, the version which the template or plugin was tested. While this will ensure you that the theme will work for your wordpress version, wordpress theme developers won’t get back to re-test the template and rewrite the readme.txt file. How important is this for you ? I would say that this is not important at all.

WordPress rolls out about 3 new major versions every year ( and 10-20 minor versions, security updates, patches, etc ). You can’t expect a theme author to update all of the their themes that many times every year. More, if the theme is free there is not incentive to update it. However, the “tested up to” field has nothing to do with the fact that the theme will work or not on your wordpress installation. Odds are that 99% of the themes created in the last 3 years will work on any wordpress version. If the theme is very old ( 3-5 years ), some features like admin panel menus or widgetized sidebars won’t be available.

In the last 3 years there were not too many improvements on the front-end part of the wordpress ( except the development of the default templates ). So, if you like a theme, use it and ignore that fact that it was not tested with your wordpress version.