When declarer first see the dummy he should analyze his contract relative to whatever contract he thinks the field will be in.  If it appears that the field will be playing in the same contract he plays the hand as he always does - tries to make the maximum number of tricks.  If however, he seems to be in a different contract from the field he must modify how he plays.

Here are some examples:
If he and/or partner stretched to find a good game contract that most of the field probably will not find he must play very carefully to insure his contract.
If he is in a suit contract, going down for 100 when the opponents can surely make two of a major for 110 he must not risk going down one more trick in an attempt to make the contract or go down a trick less.
If he is in the wrong contract he must do what he can to make enough tricks to compensate.  For example if declarer is in a three no trumps contract when it appears that four of a major will score better he must do what he can to make an extra trick. Even an unlikely line which will cost his contract if it fails.
Sometimes he must hope that the pairs in the 'right' contract will have bad luck (unfortunate distribution, finesses off-side, etc.).  In cases like this he should play for the bad split or whatever.

Remember a zero is a zero and a top is a top.   If you go down in a contract trying to make an extra trick and you can not get a lower zero and if you do not get and extra trick in a contract that is going to be a top anyway you will not get a higher top.