By mulching grass clippings, you are keeping nutrients in the soil longer. Many think it may contribute to the thatch layer since dead grass clippings may come up when you de-thatch, but is not necessarily causing the problem that are associated with thatch. The grass clippings decompose and adds organic matter to the soil, increasing the availability of nutrients in the soil. A significant amount of grass clippings would be needed for it to cause problems associated with thatch.
You want to avoid going more than 10 days, during the growing season, without mowing. By the time two weeks comes around, there may be too much grass to mulch. This would be the only time you want to bag. You do not want clumps of grass clippings! The clumps will smother out the grass underneath and could eventually kill grass. Wet clumps will make matters worse! Also, mowing when the grass is high, could lead to frayed ends of grass blades, making the lawn more susceptible to disease.