Thanks for all of the excellent advice.

I did not realize that I could give Delta my NWA number, etc. - I almost always make my reservations on-line through the airline web-site and never saw any option to input partner numbers.

I do fly both USAir and NWA, but the majority of my NWA miles have been earned through bonus programs and credit purchases. The USAir miles are 90% seat miles.

KoreanAir has been 100% earned with credit card purchases - I'll make a switch there after I earn a redeemable amount of miles. Some of the real small balance accounts reflect sign-up/registration bonus miles.

AA and Delta were flights booked for me by others; but had I know that I could use the partner account, I certainly would have.