I'll probably get flamed for saying this but here goes......
We're talking about grease and wheel bearings.
Not changing the oil in your engine.
As far as I'm concerned, grease does not normally wear
out. As long as it's not contaminated or very old it's
going to be fine.
I for one am not going to try to "flush" out my hubs every
year. Just not gonna do it. I changed bearings and seals
last spring and I'm satisfied they are as full of new grease
as can be so I'm done for a couple years.
After that I may pump some new stuff in there from
time to time but I'm not going to try to run a half
cartridge thru the fitting!
Oh yeah, bearings are "color blind"