It all depends on the model you have and the arrangement of the carburetor and gas and oil fills. Generally, mowers made in the last fifteen years or more are designed so you can tilt up the carb side to drain the oil out of the extended oil fill tube or for under-deck maintenance. On these mowers, the gas fill is always on the carb side to prevent gas spillage when tilting up the mower.
Any time you spill oil from tilting, you are potentially oiling the paper air filter and getting oil into the combustion chamber which can lock up the engine and/or cause a failure to start as well as an air filter replacement.

As Sax is pointing out, tip with the airfilter up. Other things that can help are to be ready to scrape, do it whilst the grass is still wet and rest the back of the handle onna block or something that doesn't let it tip beyond the perpendicular point. These things will allow you to get the job done quickly. The normal disclaimers such as wear leather gloves, disconnect the spark plug wire and work carefully should go w/o saying. Anytime you tip a mower, the potential is there to flood. Also, a sharp blade, higher cutting height and proper engine speed will allow the mower to hopefully get the job done properly with less clumping. Mulching when wet is tough to do with many mowers as well.

Per your email "From another site I saw this...
please? do not tip the mower with the air filter facing up, this allows the oil to seep out through the valves and on to your driveway, also when you go to start it up, chances are you will be out of pocket the money plus some more that you will have to pay in order to have someone that knows something about small engine maintenance to repair the damage that was done by doing this stupid move.
Comments please."
In the perfect world, a four cycle engine would remain in the orientation to which it was mounted. The very next best position is with the spark plug pointed into the sky. The problem with that is a typical walk behind that is tilted back on the handle doesn't give you much room to see where to scrape. The key to cleaning a mower is to get the job done quickly. Once again, the key to cleaning a mower is to get the job done quickly.
If I have a mower on it's side for 15 minutes, a small amount of oil can seep up the valve stem and some residual oil finds it's way into the muffler. It is even possible to hydraulically lock an engine when too much oil seeps into the cylinder. It becomes an inconvenience, not a part changing costly repair. Variables in engine design, wear, valve orientation when shut off can come into play, but only when the unit is tipped for a longer period of time. The only damage that could be done would be to damage the paper media of the filter. Tipping with the filter up protects the filter media from damage.
Best of luck on whatever you decide to do. You may help others by keeping the conversation on the board for all to see.

Thanks to all.
I'll report if I learn anything significant that adds to the discussion. Ron

Try tipping the mower up from the front. Hold it with one hand and clean the underside with hose. A jet nozzle will increase water pressure. I do it after every mowing and there is no grass build-up.

Unscrew the gas cap, cover the opening with a piece of saran wrap and then screw the cap back on. That should keep the gas from leaking out for the couple of minutes it takes to clean the underside of the deck. Just don't forget to remove the plastic once the mower is upright again. At least, it'll cause a vacuum to build up and the mower to stall the next time you run it, because the tank can't breathe. At worst, and more likely, the gas will eat the plastic and possibly gum up the whole works. Oh, tilt the mower with the gas tank side up.