Search and read. The value of expertise for combat builds has been discussed extensively.
I've been gearing up in the 25 heroic TOC stuff but noticing the lack of expertise on leather gear other than twopieces being [Acidmaw Treads] and [Vengeance of the Forsaken] The second being pretty useless far as spreadsheets have been telling me compared to other trinkets. So is expertise that really important for combat rogues? At the moment I'm at only 13 out of 26 for the cap, given I just replaced my acidmaw boots for the 25 heroic ones from faction champions which my spreadsheet said were a bigger icnrease in dps. Leaving me with just my crimson star as my only expertise item. I take its not bad to have such a low expertise value as combat given our finishers can't be dodged, just our auto's and SS, just feels weird to have it so low for once.