Of all the things to worry about, why should you worry about the glasses and the dinnerware? Well, maybe it's not that you should worry so much as you should make sure that you have everything that anyone could need.
Depends on the menu
If you're having something that requires special utensils, you'll have to factor that into your dinnerware plans. This sounds silly, but you truly have to understand that every detail will help your guests and you feel comfortable. You'll be eating too, so that butter knife won't go through your steak any easier than it will through your guest's.
Most catering services will incorporate the correct utensils, but you may want to double check to be sure. This is especially true when you're having a dish that is a little tricky to eat-meats, shelled seafood, etc. You want to make sure that your guests are completely comfortable and able to eat what they are served.
The same goes for drinks. You will want to have a water glass or goblet plus a champagne glass next to each place setting. If you are having a bar, then the additional glasses will be given by them. If there is wine and beer, then they can be served in the appropriate glassware.
If you're doing it on your own
There's nothing more frustrating than realizing that you haven't planned for enough people. And when it comes to dinner ware and glassware, you need to rent more than you think that you do. You've certainly been at a party and set your glass down, lost it, and then grabbed a new one to replace it. This is the same situation, only there are a lot more people.
When you're planning your own wedding and renting the dishes and flat ware, you want to plan for at least fifty more people than are coming. This gives you enough leeway should glasses get misplaced or lost.
Flat ware should also be ordered in bulk as it can easily drop on to the floor or be miscounted. When the supplies arrive, you want to count and recount everything to make sure that you have enough.
And in a pinch, you should have plastic glasses and cups ready, as well as flat ware. It's not the prettiest sight, but when you have nothing else, it will have to do.