depends ideally you need different hieghts for using power tools and hand tools.
i think the rule of thumb for traditional benches is to put your hand by your side and it should sit flat on the bencj top.
for power tools the height needs to be more.
If you want a bench to do both then apply Drew's rule of thumb. If you'd like to prove this try out working with a plane on your kitchen worktop to see how comfortable it feels. I built a dual heght workbench with hinged blocks under the feet which gives me the best of both worlds - I find a power tool workbench is about 3inches higher for me (I'm 6ft 2in). Another thing to consider (if you have the space) is an assembly bench or assembly trestles. These are about 18in to 2ft high and allow for comfortable assembly of carcases