Well, depends on the gig, is not the same for me to start playing a song in a Casino than in a Senior Dance, or in an Assisted living place....I observe the general age group, also the ethnic mix.....that gives me a clue what should song style I should play. The "New" seniors, Baby Boomers like more oldies, 60's some 50's and also some 70's, very different than older Seniors.
Also, here in the Phoenix, AZ area, there are places on the East side that like more Polkas, Old time Waltzes, and on the West side, they prefer some Rock & Roll, Night club 2 step, as well even further West, they like Salsa, Merengue, Cumbia, etc...seems like certain areas have the tendency for certain music type too.
In some local Casinos, at least on the Arizona area, there are lots of Latinos(50/50), so, I have to "shuffle" the repertoire, that is very different than lets say a Senior dance, witch in some have a "Board", with categories like Foxtrot, Waltz, Latin set, Swing, West Coast Swing, etc...you play what you want as long is from that category. Senior dancers, are picky about the tempo, so, you have to really be on top of that too.