using possibly null timestamptz columns

Given a table with a pair of timestamptz columns (lets call them s and e)
which are typically null, is there a better way to write this where clause
snippet:
   where ( s is null or s <= now() ) and ( e is null or e >= now() )

In particular I hope to ensure efficient indexes.

-JimC
-- 
James Cloos <cloos@jhcloos.com>         OpenPGP: 0x997A9F17ED7DAEA6