Should Tool Length Always Be as Short as Possible?

No. Short tooling minimizes deflection and is the right default, but there are real cases where a longer tool is the correct choice: deep pockets, tall bosses, and features that sit below an obstruction a stubby holder can't clear.

Why short is the default

Deflection scales with the cube of length for a given diameter. A quarter-inch end mill stuck out half an inch behaves nothing like the same tool stuck out two inches under the same load. Shorter tools run truer, chatter less, and hold size better on finishing passes. If a job doesn't need the reach, there's no reason to trade rigidity for it.

When length isn't optional

Some geometry forces the issue. A 2-inch deep pocket with a 3/8-inch cutter needs a tool long enough to touch bottom. A boss surrounded by taller walls needs enough stickout to clear them on the way in.

In those cases the question stops being whether to use a long tool. It becomes how to get the most rigidity out of the length you're stuck with. Go up in diameter if the feature allows it. Use a necked tool instead of a plain long-shank end mill, so only the working length is thin. Rough with a shorter, stiffer tool first to open clearance, then finish with the long one that only skims a thin layer.

What changes with reach

Longer tools need lower feed and lighter radial engagement to hold the same finish, because the same cutting force produces more deflection at the tip. Depth of cut usually needs to come down too, since axial load adds to the bending moment on a tool that's already flexing.

Climb milling helps here. It tends to pull the tool away from the wall it just cut rather than dragging it back into rubbing contact, and that matters more as stickout grows.

Toolholder choice matters as much as the tool itself. A long end mill in a weak collet chuck moves at the holder before it moves at the flute. Shrink-fit or hydraulic holders grip tighter over the same length and claw back some of the rigidity the extra reach cost.

The practical call

Pick the shortest tool that reaches the feature with margin for tool changes and holder clearance, not the shortest tool that reaches exactly. Extra length "just in case" buys nothing but deflection.

If two tools both reach, take the shorter one and accept a slower feed elsewhere if needed. If only one length reaches, plan the operation around the deflection you now have, rather than pretending it isn't there.