I have to agree with W'Boy SATCO - if you have Mode C - select it.
I assume he speaks from an ATCO standpoint.
I fly in and out of Class D airspace controlled by NATS and there can be a multitude of 7000 squawks flying around the periphory some days. Whilst accepting that the Alt. readout will only be unverified against a 7000 squawk from an ATC standpoint, I would much prefer to have some idea of the altitude of potential conflicting traffic than not.
I was descending IMC out of 5,000 a few weeks ago (outside CA) with info on two radar contacts potentially conflicting but no Mode C.
It made me look for a hole in the clagg real fast
Is there any reason not to select Alt ?