The thing is that EDICT isn't a real dictionary - it's a file that has every Japanese word that Jim Breen has ever come across (or received an email about) and every English word that that Japanese word might mean in any context. It does have an unreliably applied 'common word' tag in it that can be useful for filtering out obscure readings.
Rikaichan is just grabbing the entries out of EDICT, so you'd have to go to a web page or an application to filter them properly, I believe, although I don't know all Rikaichan's options.
Anyway, real dictionaries naturally are going to note when a reading is obscure, often referencing you to the more common reading. If you don't have an electronic dictionary of your own and are presumably tied to the web anyway whenever you read Japanese, then you can find good dictionaries at goo, yahoo, and excite, and probably some other web portals as well.
In this case, だいぶ is the only reading that appears in プログレッシブ和英中辞典
(There is another reading in the 国語辞典, but おおいた is defined as 九州地方北東部の県,
and is probably not what you want unless you're reading about travel in Kyushu.)
http://dic.yahoo.co.jp/dsearch?enc=UTF-8...5%88%86&oq=