QuotePartially agreeing. Slower does make for good practice, but when it gets to the point where a rumba is actually a Bolero (keep in mind that we taught American Rumba, not International Rumba. American Rumba is already fairly faster compared to International), dancing on-time becomes painfully difficult. Believe me, I intentionally practice my International Rumba at the Bolero speed and I would not want to have a complete beginner go through that. Same deal with the cha-cha. If it becomes too fast or too slow, attempting cha-cha makes no sense. People noticed I was doing salsa to faster songs even if they were labeled as cha-cha because there was no way I would be willing to force a beginner to move at a pace even I myself am not comfortable with. As for the sambas, I sat out completely as they were never taught and doing cha-cha so slowly would be uncomfortable to a beginner. (Note: for those who aren't aware, beginners have a tendency to want to keep moving as being comfortable with stillness during a dance is something that MUST be developed)
It does depend on the song. It'd have to be a hybrid, where the tempo's just slow enough for the slowest dance, but fast enough that the other dances could still be danced.
That said, in my experience, for raw beginners, they usually prefer slower music so they can think about what they're doing. They eat that extra time not by stretching their movement, but by half-pausing with each step to think about where the next one is. I figured more people at the ball would be a raw beginner than someone who has about 1-6 months experience (at which point, the preference in tempo does change).
And, well, I compete rhythm, and I can tell you, I sure as heck prefer a slower tempo, even though that isn't what I get. :) [I want more time to milk my cuban motion! :P]