Depends on the context and how you do them. For instance if you want all records for a given day I might write it like this in Oracle:
\n  ... some_dm >= TRUNC(:date)
AND some_dm < TRUNC(:date) + 1\n

If there is an index on some_dm, it can now be used. And sure there is a function, but a smart optimizer should avoid having to recalculate it all of the time. I don't have proof that Oracle actually does it, but I've never had a query performance problem be tracked down to this either. And I don't have tools to test it at home. (However I'd be somewhat surprised if it both could correctly spot the optimization when it let it use indexes, which I've seen it do, and not just have logic to factor out the function in general.)

Cheers,
Ben