Were past VP choices net positives or net negatives for the ticket?

[J. J.]

2008: Palin hurt, and Biden helped in PA and NJ.
2004: Cheney neutral, Edwards hurt, came off as slick.
2000: Cheney hurt, as he looked recycled, Lieberman helped a great deal in FL.
1996: Kemp neutral and Gore neutral.
1992: Quayle hurt, Gore helped, Stockdale hurt.
1988: Quayle hurt, Bentsen helped a lot.
1984: Bush helped, Ferraro a disaster.
1980: Bush helped a lot and Mondale neutral.
1976: Dole hurt, Mondale helped.
1972: Agnew helped, Eagleton was the greatest disaster.
1968: Agnew helped and Muskie hurt, LeMay hurt a lot.
1964: Miller and Humphrey both helped.
1960: Lodge hurt and Johnson helped.
1956: Nixon neutral and Kefauver helped.
1952: Nixon hurt, because he came off as slick, Sparkman neutral.
1948: Warren and Barkley both helped.

[J. J.]

1968: Agnew helped and Muskie hurt, LeMay hurt a lot.

This is backwards. Muskie greatly helped Humphrey's campaign and even would allow protestors to take the podium to air their views. Historians agree that the choice of Muskie was a good one. Agnew was a drag on Nixon. Not just politically, but Agnew was a gaffe machine and made Nixon look bad, especially in 1968 when the Democratic ads tore at Agnew's image.

Agnew did the nearly impossible in 1968.  He appealed to southerners and Rockefeller Republicans (he was Nelson's suggestion).

Muskie didn't add anything.  It was two extreme liberals and there was no real geographic base.  A candidate from the west, south (which would have been difficult), or even the midwest would have been better.