As a general rule of thumb, I usually suggest that you tip those who have provided exceptional service to you and your guests. Many vendors, such as caterers and limousine services, include gratuity in their quotes and/or contracts. But other vendors, such as the delivery person who brought you such lovely flowers and the baker who went above and beyond to design your dream wedding cake, often get left out.
Here's a handy link that provides useful information as to who and how much you should tip:
Please keep in mind that for those that aren't listed, such as family members who provided assistance, there are other ways to "tip" that may be more meaningful. A small thank you gift usually does the trick.
Here's a guide that may be useful to you:
Happy Tipping and Happy Planning!