An even sample through the soil profile to 4 or 6 inches (just indicate which it is on the form appropriately) is the correct way to do it.

Even most tree feeder roots sit in the first four to six inches.

Even though topsoil is only 3" deep, in most cases, commercial fertilizer top dressed onto the soil will penetrate into the soil, perhaps further than 3" or even 6", depending on the coarseness of the soil and how heavily the soil is irrigated, including what rain is received during the season.

I think it would be more important to know how far the fertilizer will penetrate into the soil and test the soil to that depth, whether it be 8" or 10", according to the variables that cause fertilizer to 'leech' or fall through the soil.

Also, how would one determine what parts of the lawn will be feeding only grass and how much will be feeding tree roots? Maybe one should feed the grassy areas separately with light, multiple applications of high N fertilizers, to prevent deep fertilizing? And feed the areas containing trees differently, with to what trees need and less often?

That does tend to make the reading of the soil sample's report extremely difficult as the typical profile used is to six inches (maximum). Four is also a standard. And while structural roots do reach down there, feeder roots generally don't (except in extremely coarse soils where the oxygen levels that low are acceptable).

Trust me, use one of the standards and run with it. Out of hundreds of soil tests, I've never seen that fail to produce a good lawn (and gardens, for that matter).

I'm fine with it. :-)

I don't fert the lawn and my young trees are in mulch circles and they may get a handful of high N very early in the spring when the ground thaws.

Larger trees I don't worry about (I have a sandy loam with 30% silt and near zero clay).

Things here (trees & shrubs) seem to grow just fine with water only but when we vegetable gardened, we added OM to the mix.

There's certainly no issue with not feeding or feeding shrubs or trees, dealer's choice. I heavily fed some Thuja I wanted to grow and block an annoying neighbor. I mostly ignore the pin oak out front, but don't avoid it when I feed the lawn.

Trees and shrubs tend to have wide root systems that can draw resources from large areas. Grass plants have much more limited root volume. Garden plants and small shrubs are in the middle, and both blooming plants and grown vegetables are more demanding of resources than either grass or most trees.

Everybody likes a good soil, and I zoomed those Thuja to 18 feet tall in five years. Now that they're grown in, I stopped and they added maybe ten inches this year. But if I hadn't, they'd be perfectly-healthy, eight foot tall specimens at this point that would happily grow on as the years pass.