In my post-PPL and pre-CPL days I used to go as high as was practical and possible. More efficient, and with a single engine it gives you a lot more thinking time if things go bad.
Usually I was limited by not carrying O2, or by the climb rate of the aircraft depending on said a/c (on one flight with a friend next to me we'd departed at MTOM and at 11,500ft we only had 50fpm climb).
If cloud permitted I'd usually go up to 8000ft, took around 10 minutes from takeoff to get there, and it meant I could lean the mixture out enough to get a fair increase in endurance and range... all about the fuel economy
Had been up to 18,000 in mountain wave in my friends Duo Discus, and that was
cold with no heater, I knew it would be so I'd layered up a lot, but still, wouldn't like to spend long up there.