Just based on my own experience in the past (my current layout doesn't have grades, decided since getting back into the hobby didn't want to futz around with them, when I do the inevitable rebuild I'll add grade interest). I originally wanted multiple levels (still do!) but I wanted to connect them, and to do so given the relatively small size of my my layout would have made the grades too large (I don't have room for a helix). To me grades are kind of like the diameter of curves you use, larger diameter curves are like keeping grades smaller, they will allow you a lot more flexibility. Smaller diameter curves limits the type of equipment you can run, for example, while heavier grades will limit train size and what engines can handle it. On a small layout grades may be less of a problem IMO, since on a smaller layout you tend to have smaller consists anyway, to avoid the 'nose chasing the tail' scenario
One of the reasons Lionel created magnetraction as far as I know was to increase pulling power, especially on grades. Like on real railroads, maximum grade is a function of what is being pulled and what is pulling it. Can you get away with 6% grades on a layout? Sure, if you aren't running long consists and your motive power has the traction and power to do it, why not? The old graduated trestle sets weren't that far off from 6%.
On mainline railroads they kept grades as low as possible for that reason, lower grades meant longer consists and also not needing to use helper engines (which became less of a problem when they went to diesels and MU's became possible). Not to mention fuel usage, running on more level terrain=less fuel usage, less wear and tear on the brakes on downgrades, etc. Narrow gauge lines ran some pretty heavy grades I recall, but they also tended to run smaller consists, too.
