I usually perfuse the animal via left ventricle with saline (40-50ml) to exsanguate the animal (SD Rat). I have to evaluate neutrophil infiltration in lungs and brain.
Ideally Yes.. As one may see a lot of RBC during histological examination if not perfused, (alternate way is to rince the tissue properly in MiliQ water followed by PBS).
Perfusion may not be neccessary for brain histopathology (only for H&E)
For clinical histology we don't perfom perfusion to our patients before surgery.... It is just immersion fixation in NBF of sufficient duration. The RBC don't really matter for HE-staining.