You don't have to use FL if VFR but I always do because most of the class A airspace (airways etc.) are normally a FL so it ensures you won't bust them. I'd always plan to try and fly at FL50 or above on a reasonably long x/c because you have less traffic to worry about up there and comms / radar are better. I'd always aim to fly a quadrantal level but sometimes that isn't possible due to the weird class A airways in the UK.
The TA is normally choses at an alt above the highest terrain around, so in the USA is is 18,000. IN the UK not much extends above 3000', though I reckon it would be sensible for Europe to have one common TA which is based upon the highest of the Alps.