I usually hi and lo pass guitars differently depending on the sound, but as a rule, i usually just remove un-needed information. so slowly move the hi pass up until I hear a noticeable difference in sound, then i move it back slowly until i notice the difference go away, and leave it there. usually between 60 and 80 hz.
same for lo pass, bring it back slowly till you hear audible freq. being cut, then stop and bring it forward slowly till those freq. come back, and leave it there. usually 10-12 k
basically your just removing the "ghost" frequencies that are just mudding up your mix and are completely unneeded. :Smokin:
not "shape the sound" in any way