What kinds of decisions should members of congress be willing to anger their party leadership over? on what kinds of issues or decisions should they be willing to go against what they believe is the will of their constituents?

Theoretically, members of Congress should never go against what their constituents want or believe. Our system of government is a representative democracy where all elected officials represent their constituents, but the party whip in both houses is extremely powerful. Many times elected politicians prefer to act upon decisions made by party leadership instead of considering the well-being of their constituents. Politicians are all very clever and usually only look for their own personal interest, they only side with ordinary people when they seek their votes.