How you prepare your soil depends on what kind of soil you have. If you have heavy clay, I'd dig deep, much deeper than the plant will go in a coulple years and loosen the soil. I wouldn't amend, though I would plant the trees high (clay soils tend to drain poorly and also can pool holding water and drowning roots).

Personally I do little in the way of preparation this time of year, I do compost and pile my beds high with leaves in the fall. I add leaf mould (rotted leaves from the fall before) and mulch around the new planting (2-3 inches deep and away from the stem) just after planting. Digging in the wet-cold soil (too much) only serves to damage the structure of soil and leave it clumping and hard. Wait til the ground warms a bit, if you ordered from a good place they will not ship until it is appropriate for you to plant anyway. (for me, also zone 6, that will likely be around the end of April/beginning of May). Anything that comes in before that will get 'heeled in' in one of my vegetable raised beds (any good soft soil will do) or in a pinch I'll pot them up in 2-3 gal containers and place in a protected part of the yard.

If you start digging too close to young trees you risk damaging roots (ignore the myth that the roots of a tree are a mirror of its top, imagine instead the roots being more like the tree's shadow, only extending about 1-2 feet below the surface (there are exceptions of course)).

If you plant a grape vine at the base of a cherry, it will quickly overgrow and smother the cherry. Also, sunlight is necessary for the ripening of fruit and a grape in a cherry would only serve to increase the chances and severity of disease in both plants.

~Chills

The soil in my area is a heavy clay, fungus infected challenge, so I have had better luck starting the bare root trees in good potting soil in pots, then planting them in a raised bed once they are doing well in the summer. It all depends on your soil conditions and the cultivars of fruit trees you select. The website below has lots of good information on planting fruit trees. Good luck.

Here is a link that might be useful: Planting Your Backyard Orchard