I have always been nervous about the warm up screen running off the tube face on Sony projectors. I don't want to work on a client's machine and then have them later wind up with a possible tube implosion due to the warm up screen. If the warm up screen were to cause an implosion, I don't want a client to think that it was the result of something I did while working on the machine. Because of this, I always turn the warm up screen off on my client's projectors. I also keep the warm up screen off on my own G90.
The point of the warm up screen is two fold however. It does allow the projector to warm up and minimize convergence errors when you sit down to start watching. However, you don't need a white raster for convergence to come back into alignment. If you just run the projector with a black screen, you will find that in 20~25 minutes the convergence will be close to perfect with just a black field.
The other job of the warm up screen is to blast the phosphors with a full 100 IRE bombardment of electrons. It takes the phosphors about ten or more minutes of this before your black level, white level, gray scale, and gamma will be correct. If you look at a black level test pluge right after turning on the projector, you will notice that it will be much too dark compared to a fully warmed up machine. So if you start watching without any warm up, the image will be too dark and the gamma is much too high resulting in loss of detail and a punch-less image.
Full 100 IRE white quickly gets the phosphors ready for reference performance. Just watching a movie for twenty minutes does not do the same thing. Joe Kane used to point out that if you watch a two hour movie without a warm up screen, the phosphor is at its peak performance at the very end of the movie. In other words, there is not enough of a work out with regular viewing, so it takes nearly two hours before the phosphor is ready for reference use. If you use no warm up, you only see what your projector can really do after two full hours of viewing
What I do for myself is to turn on my G90 and let it run with no input and no warm up screen for 15~25 minutes. At that point the convergence is perfect. I then turn on my Lumagen with a full 100 IRE white field and let it run for 5~10 minutes. With this combo I get full performance without the risk of running the warm up screen off the face of the tubes.