It's best not to rag an engine from cold, especially if it's literally just been started up. However, I don't believe in a hard and fast rev limit. If you have to pull out smartly to be safe that's more important.
What is and isn't bad practice is hard to say, and different people apply different rules. I personally don't use full throttle until an engine is warm (I perhaps use up to half / two thirds, which is plenty for a V6 anyway) and I will limit myself to mid range rpm except in cases where I need the acceleration where I will briefly go to perhaps 3.5k.
Depends on the engine too, of course. The above is more than enough in a 3.2 Omega but I might be less respectful of a 1.0 Micra!
What is important is that you don't give it a total spanking when cold. Before I give a car full throttle and full revs together I think about how long it's been running (it's also worth a glance over the gauges anyway, to make sure all is in order).
Oil temperature is the important variable and it can take the oil a little longer to heat up than the coolant, so I probably give a car 10 minutes at full coolant temperature before I use 100% of the available performance.
Others will have different ideas, and let's face it, none of us can look into the engine and see what's going on.
Kevin