Picking coins for grading

I would say value... Im not going to send a coin that is worth less than the grading fee's. Oh also for selling it again, People tend to buy coins in slabs to make sure their genuine

And they are a real pain to get out of the slabs too

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.