I've got a 905 and I bi-amp my Monitor Audio BR5's, yes it's definately worth it.
If you are only using your system in 5.1 mode, i.e. 2 fronts, 1 centre, 2 surrounds and a subwoofer there will be 2 channels of amplification going to waste on your 905! Bi-amping you're front speakers will provide seperate amplification for the treble/midrange and the bass. I've found it noticeably better.
Just think of it as connecting 2 pairs of speakers.
1. Remove the metal links between you're speaker binding posts.
2. Preferably use a bi-wire speaker cable, e.g. QED Silver Anniversary Bi-Wire, connect the Front right and left 905 binding posts to the treble/midrange (top set of binding posts) speaker binding posts and the Surround Back right & left 905 binding posts to the bass (bottom) speaker binding posts.
3. Enter the Speaker Set-up on your 905 and change the setting from normal to Bi-amp for speaker A.
This won't damage anything assuming you wire it up correctlty and have removed the binding post links on your speakers. You should notice improved treble clarity and a faster deeper bass. If you want to experiment, the Front R&L outputs on your 905 will probably provide better quality amplification than the Surround Back R&L outputs, you may get better bass by connecting your bass drivers to the Front R&L and the treble/midrange to the Surround Back R&L, you choose which sounds best.
P.27 of the Onkyo manual shows you how to do it.
Good luck.