For the host, not hearing back from you can be a vexing problem, particularly if it’s a smaller gathering. If you’re not sure you can come, or will be late, most hostesses would prefer you tell them that rather than not responding at all.
If you’re planning a party and wondering whether or not to include an RSVP request on your invitation, it is completely optional. Certainly for a formal dinner you will want to know how many places to set. In contrast, an RSVP might not be necessary for an open house, since people will be coming and going throughout and food is usually more casual.
Seasoned hostesses are good at estimating quantities and are always prepared for surprises. As a general rule of thumb, however, it’s a good idea NOT to be the one to surprise her!