To be honest I don’t think my lips ever “stiffen”, unless I have been doing a TON of playing. I also think the weight analogy doesn’t really fit.
As for being able to respond at any time I would agree. I practice it! For two main reasons:
a) weddings
b) Orchestral work.
While I would hardly consider myself a professional trumpeter I need to sound good when I play weddings or with some of the regional orchestras here in Georgia. I cant tell you how many times I have sat in a cold church for 30 minutes, or more, then had to play piccolo, or counted 100+ rest to have to come in on some exposed note. In my own playing I can play things cold that I have truly mastered, things I can’t play with out a warm-up are things I would have trouble with warmed up or not.
For a little test try this:
Do some sight singing first thing in the day before you play. Sing about 3 or 4 “tunes” .
Then pick up the horn and play!
With the brain already engaged don’t you find it much easier? I DO!
Or put in a CD of an orchestra and pull out the part. count the rest then come in! if you have enough foucs or a clear enough picture of what needs to happen it really isnt a probelm.
Now I am not saying do not warm up, and I certainly value a good warm-up at the start of the day myself, but I think after that it is all not needed.
http://www.chisham.com/tips/
Listen to the Arnold Jacobs master classes for some more info on warm ups.