That is actually a very interesting question and it turns out that whether it's better to fanout on write or on read depends on a few different things. There's a very widely read paper on the subject you might enjoy:
Feeding Frenzy: Selectively Materializing Users’ Event Feeds Authors: Silberstein, A.; Terrace, J.; Cooper, B.F.; Ramakrishnan, R.
A quick Google should turn it up for you somewhere.