Err.. Sentence Completion ?? You mean Sentence Correction right ??:)
If you search on the web and elsewhere in this forum you would get some documents covering GMAT grammar.. I read some documents earlier but didn't give much attention to it.
Reason: Grammar is only one area tested, there are others - parallelism, idioms etc tested in the exam. If you are making SC errors solely or primarily because of grammar mistakes, then concentrating on this area might help. But again if one is weak in say idioms for example, mugging up all the idioms available out there is just not worth it. So you need to strike the balance between what to cover (and upto what extent) and what to ignore .