Oh just to add - with lentils, however long you cook them, you won't be 'boiling good stuff away' as there is no post-cooking water to rinse away and lose the good stuff. It's more like making porridge.
Wash before cooking, as mentioned by IaninSheffield. I just put mine in a sieve and run the tap over it then swish the sieve in the bowl of water and let it drain. Even if you're not pre-soaking, wash all pulses before cooking, whether you're putting them in the pressure cooker, or an overnight slow-cooked stew.