As silly as it may sound to some, my experiences are very similar to Albert's. My current MC cartridge (shelter 501) benefits from about a side and half a flip side before settling in to serious listening. I don't recall facing this issue with several of the MM carts I had, but memories for some things are short! :-) I know that a side or two is worth the wait with the Shelter....
Do cartridges need warm-up?
Just curious: I know that my phonostage, pre-amp, and amp (all tube) sound best after a couple of hours of warm-up. It seems like my cartridge also likes to play a few LP sides before it starts giving its best. I'm wondering if I am imagining things or if this is a known phenomenon. I'd be interested in what other 'goners have observed. Thanks.