(This was previously a comment due to too low reputation and highly active question; now either my reputation is higher or the question is no longer highly active, so moving answer to comment.)
Consider this: what way do you tip the lawnmower when changing oil? It's the oil cap downwards. So that's the safest way to tip the mower. In the case of your mower, it would also mean spark plug downwards, but in other mowers may mean spark plug upwards. But the oil cap side is always down. You may also be able to tip the mower nose ("OHV") up, handle down, if the handle has a quick release mechanism -- read the manual to see if this is possible. Without quick release, it wouldn't be tipped 90 degrees but only 45 degrees that way, and it would be really inconvenient then.
Also theoretically it's possible that if the mower is tipped for very long amount of time, that too much oil would get slowly past piston rings to cylinder (hydrolocking it), and past open valves to intake, ruining the air filter. To eliminate this theoretical possibility, you can pull the starter cord for few engine revolutions to test its resistance, and then pull again and leave it at the point where you feel the highest possible resistance. This means the piston is at top dead center, so oil getting past piston rings won't hydrolock the cylinder, and the little oil that fits into the cylinder won't get past open valves to ruin the air filter (because the valves are closed in that position).
I think the hydrolocking and open valves problem is not likely an issue in most cases when tipping the mower oil cap side down. It would be a problem probably when tipping mower nose down (which is not recommended due to that potential problem). But it could be useful to learn to get the piston to top dead center everytime you tip the mower, just to be safe no matter what way you tip it.
Never tip the mower air filter side down (oil cap up). In this orientation, oil gets to breather tube, from which it gets to air filter, soaking it with oil. For paper air filter elements this is really damaging. Oil could also drip away from the mower, and also the mower could smoke when restarting it.
So, as a summary, of the four ways to tip a mower, two are good and two are bad.