Depends on the situation.  If I know high alt buff types are in the area, I climb up to them.  If I know a base is getting swarmed by a bunch of low alt stuff, I come in around 7-10k or so quite often.  The longer I am in the air unmolested, the higher I get.  Can't remember who said it originally, but it's kind of true that if you're flying and not climbing, you're wasting time by not storing energy.  Doesn't have to be much.  If I want to get to the big red bardar faster, I'm quite often only climbing 500-1000fpm, but just constantly banking that E, no matter how little at a time.
Wiley.