I would suggest buying in at the table max as it does a couple things. It allows you to win more money when you have a good hand and can get all your chips in, specially if you are the effective stack. Second it allows you to play more hands. What I mean by this is that you should only be playing multi-way pots with speculative type hands (suited A's, suited connectors, small pair) for less than 5% of the effective stack size. If you buy in for a small amount, then most times you will not be able to make these calls profitably as they are more than 5% of the effective stack size (which would normally be your stack size). The goal of these types of hands is to invest very little with the chance to win a large pot multi-way (ie flopping sets, straights, flushes etc. and only continuing with two pair or better or a draw of 8 eights or more).
Example would be a live $1/$2 game. UTG raise to $10, MP calls, you are in CO with pair of 4's, the effective stack size would need to be $200 for you to call this hand speculatively based on the 5% rule. If you buy in for less, then you should not make this call if you can not win $200.