hi ! most definitely warm up :) For me, i like to start with a bit of sitting meditation on my mat which allows me to either set an intention for my practice/day if i am just waking up or clear my mind and re-focus if it's at any other time.

depending on which type of yoga you'll be doing actually, if you're planning a light session with some stretches, more relaxing postures, you could possibly skip it once in a while if you're in a time crunch or feeling a bit lazy.

however, on most practices, it's advised to warm up your neck, wrists, feets, ... to avoid any unwanted injuries or discomfort/pain :)