Operationally, 6.25 volts is just fine. I've built a fair amount of tube electronics over the years and I don't ever remember measuring exactly 6.3 volts on a tube's heater. For most tubes, anything within 10% is acceptable, so that means anywhere between 5.67 and 6.93 volts is OK, but I prefer to be within 1-2%. Which you are.
As PB pointed out, the precision of the dropping resistors plays a role on what voltage you get. So does the AC voltage from your wall outlet. Nominally, it should be 120 volts where I live (the U.S.), but it's common for many of us to see it a bit below or above that. I imagine it's the same in the rest of the world as well.
