What will happen if I run the heater of a CRT at 3.8Vac instead of 4.0 Vac as directed by the manufacturer?

We use these very high resolution monochrome CRTs in flying spot scanners. I'm interesting in extending CRT life but not interested in physically damaging the CRT or diminishing its usable life. I understand that running a heater (filament, cathode) at lower voltage will lower electron emissions slightly and thus lower CRT brightness. When running between 3.8Vac and 4.0Vac the brightness change in the output of the CRT is negligible. If you need more information I'll provide it.