Not sure if this is common sense because countless people do it and it irritates me to no end.
-Every single boss in WotLK drops a rare or epic item.
-90%(Ish) of groups will have an enchanter
-not everybody will need a boss drop. If you don't need it, greed it.
-In pre toc heroics, next door to no-one will need a rare boss drop. If you don't need it, greed it!
-Large Dream Shards are averaging 3-5g each
-99% of all boss drops will vendor for more than 3-5g
-For the love of god; If you don't need it.. GREED IT!
Do the math. Stop disenchanting boss drops.