But even so, today you can count on about 1/2 your mining earnings to be eaten up by electricity costs (depending on your rates, of course). Using $0.085/kWh as an example, consider this:
SP20 @ 1650GH/s, 1250W: earns $4.15/day. Costs $2.56 in electricity. Net profit: $1.59/day.
SP20 @ 1200GH/s, 640W: earns: $3.02/day. Costs $1.31 in electricity. Net profit: $1.71/day.
Obviously in this case it is better to underclock your SP20 (or SP10); you will increase your net profit. YMMV depending on your local electricity rates.
I think most people have higher electricity/hosting rates than that, so underclocking will be even more important.
Only a few places in the world can accommodate the current price even when underclocked. WA at a little over 2c per kw for example.