I generally make that decision based on value or at least potential value. Like coincollect1 said it doesn't make much sense to send a coin in that is worth less than it costs to grade it. I added in potential value because sometimes a coin has large price differences between grades such that the coin in the lower grade is not worth too much but the higher grade is worth a lot. If you feel confident enough it may be worth the gamble. Generally the coins that need authentication are the ones that are valuable enough to make counterfeiting worthwhile.
Quote: I never do and never will. I can not put slabs in my Albums.
I agree. I would never voluntarily send a coin in for grading. However, I can see myself buying a slabbed coin if it what I am looking for. The security is an added benefit, but not a replacement for my own diligence.