Alternating sides during feeding

Been breastfeeding for over a week now with my son.  I'm new to breastfeeding and was wondering if it was necessary to offer both breasts while nursing.  He tends to fall asleep so I don't always get to offer both.  I do burp afterwards and he usually stays asleep.  In those cases I just start with the other boob at the next feeding.  Is this okay?  I don't want to end up with lop sided boobs ;)