With old cars, the problem lies more in when the last time it was done. Often times with older cars they aren't cared for as well as new cars. And unfortunately that may cause some damage, or at the least cause you to have to replace a lot of stuff at one time. Just keep up with the maintenance and you'll get plenty of life out of the old girl
For the plugs, not just check them, replace them. Always a good idea every year to replace them. Grab a set of AC-Delco, they came from the factory with those, and they are by far the best. The AC-Delco model number is R45LTS6. Gap them at .060" Also grab a set of new plug wires while you are at it. I personally use the Beldens, and love them. They are available at Napa. Be sure to get them in 8mm, and you may need to get it for a '93 Bonneville if they don't have it for your'*.