I'm a social dancer and have read this discussion with interest.
Most music that is suitable for dancing the cha cha either has the forth beat split into two half beats, i.e. 1-2-3-4& (traditional cha cha music) or it has both the first and third beats split, i.e. 1&2-3&4 (many popular tunes). In the first case, dancing 2-3-4&1 fits and a lead in step on 1 at the start of the dance is needed. In the second case, by ignoring the split on the first beat, we get 1-2-3&4, so no lead in step is needed. There are very few tunes that are intrinsically 1-2-3&4.
I've listened to a dozen versions of Tea for Two and most of them seem to have the second rhythm, namely 1&2-3&4. So I would dance the cha cha to those starting the chassis on beat 1 without a lead in step.
As for the rhumba, both of my teachers use the QQS rhythm on 2-3-4-1 (with the slow on starting on 4 and a hip movement on 1). But they disagree on the lead in step. One advocates a lead in step on 1, which is what I've always done, but the other teacher "allows" that for beginners but says that in the rhumba one never steps on 1 and he suggests doing a lead in step on 4, with a hip movement on 1. That to me seems to defeat the whole purpose of the lead in step, which I've always thought was to satisfy the natural desire to always start a dance on 1. If one is not supposed to step on 1 in the rhumba, then one might as well forget the lead in step all together and start on 2, ideally with a hip movement on 1, rather than starting with a lead in step on 4.
