It may seem a little ironic: first I told you not to show up early  to an event and now I'm highlighting the importance of not showing up too late to a party.
This rule should be remembered and practiced for dinner parties especially. Your hostess has taken the time to create a meal and has specified it will begin at a certain time, so be there and ready to eat at that time. A dried out roast chicken and cold pasta are what you have to look forward to when arriving late to a party, and you are not only taking advantage of your hostess, but you are disrespecting your fellow guests. If you are running late - for whatever reason - you owe the hostess a courtesy call to let her know you will be there after the party's start time. Showing up more than a half an hour late for a dinner party, and an hour late for a cocktail party without a quick phone call is unacceptable. If you come to a party late without calling, trusting that the hostess is exceptional, do not expect to be invited to more of her parties in the future.