Starting points should vary with every song you produce - each one must be considered if indeed it even needs EQ and why?
Should what you're correcting be fixed in the mix prior to mastering?
It's much better to apply gentle, broad curves in a mastering situation - anything more drastic definitely points to a problem in the mix and should be corrected there before it gets that far.
On saying that - one plugin I use a lot is the TRacks3 Linear Phase Eq, but some of the ones that come with Sonar are well capable of doing a good job in this role, especially their take on a Linear Phase, the LP64