How often should/do you wash your baby's hair?

Jacob really hates having water splashed on his face which makes washing his hair a bit awkward. Four nights a week one of us has a bath with him, the other nights, he has a bath on his own. The four nights where one of us is with him, it's generally ok to wash his hair, but the nights where he has to sit in the bath on his own, he positively hates it. I think he has become used to one of us being in the bath with him, so he really objects to being in the bath on his own, and objects even more when it comes time to wash his hair on those nights!!!

So I'm just wondering, is it necessary to wash his hair every day, or can/should it be done less frequently (I just thought it might avoid a few tears!). It's not like he has loads of hair to wash after all!!