I did not open the bleed screws when I replaced my pads. However, I did take the truck to the dealer and have them bleed the brakes before I replaced the pads. This removed the old brake fluid from the lines and calipers.

Brake fluid gets "dirty" from 1) contamination entering the reservoir and 2) being in service past it's recommended life span. That's why Nissan recommends fluid flush every 2 years.

Contamination (including rust) can damage the valves and valve bodies in the ABS modulator (aka ABS control unit/pump). I admit, it's a long shot (it would take an awful lot of crud to cause damage, plus you would need to push an awful lot of fluid back through the modulator), but it could happen. Better to be safe than sorry.

See my web page for a front brake how to: Xterra Service List