It can depend on the bra. Frequent washing will make them loose shape and elasticity faster. If your bras are really expensive (I know a lady who has these HUGE boobs. I mean Dolly Parton's look small next to her. She has to special order these 80$ bras) don't wash them too much. Hand wash. Though you can rinse them while wearing them to keep smells and body oils from getting to be too much. Since so many first world people bathe so much, its not necessary to clean them as much. Though panties do need to be washed each time, since you do "ooze" (that's what my friend calls it.)
*gets kidnapped by FBI*
*sobs* single guy