With today's mowers and the different mulching blades available there really is no need to bag. It's a common misconception or belief that leaving grass clippings on the lawn encourages thatch and it really doesn't, only when or If you let your grass grow too long and need to cut an inch or more off the lawn should you consider bagging or removing the clippings.
If your grass is cut on a weekly schedule with a good set of mulching blades you shouldn't ever need to bag, a good set of mulching blades will cut your grass 4 or 5 times before being discharged into very fine pieces and they're 80 to 85 percent water and will break down very quickly leaving nitrogen which is beneficial to the lawn.