Does it make sense to "collect" smaller transactions balances into a single account during low activity? My understanding is that it's possible for extra bitcoins to be left in several accounts within my wallet.  The scattering of these coins into different accounts may cause future transactions to become larger in size (Kb), and therefore more costly in fees.

Does it make sense to prematurely consolidate these small accounts into a single "spend account" at some point? (perhaps while I sleep?)
In satoshi client, the "accounts" therein are only a convenience for labeling of receiving addresses. They don't matter for sending at all.

If you receive bitcoins, the balance is not added as in bank, but the amount is saved as "unspent output". Client only shows all these outputs together as one available balance. It doesn't matter whether one address has n unspent outputs, or these are on more addresses. When you send bitcoin, the fee is based on number of these outputs used and it is higher if they are very recent. So trying to consolidate them won't help much, only that you pay the fee sooner. And it isn't done "while you sleep", it's done by simply sending your whole balance minus fees to yourself.

In other clients, the fee algorithm can be completely different.