When ordering, ask whether there's a "transport kit" option. That will include specific places for tie downs.
I don't have a chair with a transport kit (it wasn't available for my chair when I bought it), but I do have a rigid manual wheelchair and have used it on buses all over the place. I use the back axle tubing and then the tubing the front casters are attached to (or the tubing that connects my footplate because I have a center-mount rigid footplate). There's also a rigid bar across the back of my chair that I've used occasionally, but I prefer not to because it's high up so too tight and it could tip me over. Absolutely do NOT let them attach them to the back wheels. I haven't had that issue as much at WDW, but other public transit drivers seem to gravitate to those because they're easier to reach than any of the solid tubing.
There are rigid manual wheelchairs where finding tie-down spots would be harder, but even looking at ones like the Icon, I can find spots that should be safe. Anything from the major manufacturers (Quickie, Invacare, TiLite, etc.) should be pretty easy to find tie-down points on.