So, years ago, I built up a repertoire of about 60 songs, the great majority of which have middle C or above as their lowest note.

The ones that went below (such as Johnny Todd on the Dig Trad Mirror version) I transposed and checked with a better musician than myself (Julian Sutton, in this case), such that middle C (or just above) was the lowest.

Thus, if I get the fingering correct on my tenor, it sounds close to how I should sing those songs as a tenor and, thus, is ideal for intros; but, if I (rarely) pick up my plastic alto instead, I think it's okay with the very same fingering as the intervals between the notes are still the same and I'm playing solo - as distinct from "playing all the right notes not necessarily in the right order" (Eric Morecambe)!