I don't engineer batteries, so I don't have any insider info, but as a technophile I've done a bit of research on the topic.
Lithium-Ion batteries, such as what you find in the Sony Reader and iPods, do have a limited amount of full charges. The key word, however, is full. As many people with old iPods will tell you they still charge and still function. They simply hold less of a charge than when they were brand new. After 300 full charges you may find yourself only getting 60-70% of your original battery life.
The main factor in determining how well your battery holds is not whether you wait until it's fully drained or choose to keep it topped off. It's heat. If topping off the battery keeps it cooler to the touch when it's charging, then do that. If it doesn't make a difference, then it doesn't make a difference.
Charging 10% of a full battery counts as 10% of a full charge. If you should get 300 full charges, then you should get 3000 10% charges.
Again, the months and years of heat exposure will affect battery life the most. And don't worry about that 300 count. You're device won't stop working on charge 301
