This question has some information about when to retire a rope. The core of a rope doesn't become weak from top-roping or from sustaining lead falls with a small fall factor. It becomes weak from sustaining multiple lead falls with very large fall factors, approaching 2. The only way to get a fall factor greater than 1 is if you fall past your belay station (e.g., in multipitch climbing with a hanging belay); if you've never fallen past your belay station, then you have probably never strained your rope to the point where the core was weakened by any non-negligible amount.
I would hate to use a rope that has not experienced any or few lead falls, as it is still a rope I would prefer to save for actual leads.
This isn't a correct way to approach the problem. Letting a rope catch some top-rope falls will have zero effect on the rope's safety or usable lifetime. If you've got a new rope anyway, there is no good reason not to use it.
To put this all in perspective, a modern climbing rope is ridiculously strong. It's essentially impossible to break one in normal climbing use. When people put a climbing rope on a test machine and stretch it to the breaking point, a brand-new rope typically stretches to about ten times its normal length before breaking. Try to imagine what that would look like: the rope would look like taffy being stretched so much that its diameter was about 3 mm. The UIAA has a "standard fall," which is a fall factor of about 1.7 with an 80 kg weight. This is a fall so violent that if you took one in a trad-climbing environment, you'd probably rip out or destroy your gear, your belayer might not be able to keep the brake strand from slipping, and you also might get vertebrae broken by the force of your climbing harness stopping your body. A rope has to be able to hold multiple falls of this type in testing.
People have in fact tried to test empirically whether ropes get weaker under normal use -- as opposed to abuse such as very violent falls. See Smith 1988. With static ropes that had been abused, they were able to detect some weakening with use. But we're talking about dynamic ropes that haven't been abused. With dynamic ropes, they were generally not able to get the rope to break when they put it on their test equipment. The general idea is that materials have an elastic limit, and when you subject them to a strain that's less than the elastic limit, they return to their original size and shape when the strain is removed, and there is no damage. Smith's general conclusion is that "Care and storage is the strongest factor that influences rope strength, far more than age." This seems to agree with Johnson and Klonowski's finding that dirt and sand can significantly reduce the strength of a rope.
A more realistic worry with an older climbing rope is that it can get damaged by rock fall or by rubbing against a sharp edge.
Also, as pointed out in Mr. Wizard's answer, you should retire the rope if the sheath is wearing out. If the sheath isn't in good shape, the core becomes very vulnerable to damage.
So in summary, here are some things that significantly weaken a rope. If one of these happens, you want to retire the rope. Repeated top-rope falls are not on the list because they're not a serious concern.
serious abrasion or damage to the sheath
repeated lead falls with fall factors approaching 2
damage to the core (possibly invisible) from rock fall
possibly dirt and sand infiltrating the rope
exposure to chemicals such as fabric softeners
other types of abuse such as using it to tow a car or leaving it outside for a year
Johnson and Klonowski, "Effects of Abrasive Particles on the Projected Fatigue Life of Nylon Climbing Rope," http://digitalcommons.calpoly.edu/matesp/42/
Bruce Smith, "Aging rope," in Nylon Highway 25, January 1988, p. 3. http://www.karstportal.org/FileStorage/Nylon_Highway/NH25.pdf