if its just for your own collection, then send whatever you love. If you plan to sell now or later, most stuff in the 80's/90's would only be worth it was totally flawless, unless its a major key comic. To gauge prices of slabbed sales, you can look at GPA (paid service), ebay COMPLETED SALES, and sales data from other auction houses. For all comics, you should probably make sure the comic is worth noticeably more graded than the relializable value + grading costs (make sure to include shipping costs too), and leave a cushion for not getting the grade right.
For example, if you have a 90's book that you can sell right now for $10, but sell for $60 in CGC 9.8, you have to weigh in the grading fee plus shipping (there and back), maybe about $25 total. Plus there's risk of damage in transit and risk of getting something below a 9.8, where you'll be lucky to break even. Plus the opportunity cost of the $35 for 4 months while you wait to get the book back from CGC. So you've invested $25 for a chance to make $25 more. Which isn't awful, but your ROI is 2:1 at BEST on that $25, when it could have been higher than 3:1 on your original $3 investment selling for $10, a million times faster with much less risk. But its up to you and your own preferences and financial situation.
Good luck, and maybe don't submit ALL your books the first time out. Maybe 4 or 5 so you can get a feel for the grading standards.
Thanks Revat, I think I might try and get some of my favorite 12 cent books graded then. I was really into Kirby stuff when I was a kid. I put one of the Thor books in the PGM section. Figure if I get all the silver age omnibus books I wont need to read my comics anymore.