An interesting question.
The distance the z axis must travel primarily depends upon how deep you want to cut.
However, in the most part you might need to cut thin pieces and only have a need for cutting deep very rarely.
This creates a dilemma as the Z axis will be extended to its max for cutting thin pieces and when it is extended it is likely to be least rigid and therefore least accurate.
So, in effect, if you build for a 6" deep cut to be used once a year and for the other 364 days cut shallow then your machine is working inefficiently for most of the year.
I think the best way to solve that is to have a variable height table so that shallow cuts can then be done without the z axis extended. When the z axis is not extended the machine will likely be more rigid than when extended.
I hope this adds useful information for you to determine how to design your z axis.
Andy
Drat, imperfection has finally stopped working!!