The only cards that I would grade would be the following:
Pre War (Goudeys, T206, etc)
Pre 1970 cards of Mantle, Clemente, Aaron or Mays
Iconic rookies (Rose, Koufax, Reggie, Nolan Ryan, Seaver etc.)
I'd grade the above in any condition, because a grade will enhance their value, even if they are low grade. I wouldn't grade anything after '70 unless you are talking about a Jordan or Gretzky rookie, because those two are heavily counterfeited, and if authentic, they will bring decent money even if they are beat.
I may grade other cards that appear sharp before '70, but it's a real crapshoot. Unless you have an eye for it, you will likely be disappointed. For most cards, outside of those I mentioned above, unless you get an 8, 9 or 10 (9's and 10's are extremely tough with vintage Topps based on centering alone), grading is not worth the trouble or cost.