How important is increasing cover crop mass when timing burndown?

The best way to manage a lot of biomass prior to planting is to ensure that you give yourself enough time for your burndown to work. Typically it's going to take longer than it would with less biomass so plan on adding an extra week to your normal burndown schedule. As far as benefits, the more biomass that your covers produce, the more organic materials and carbon you'll be returning to the soil. The problem lies less in letting them get too large but rather not managing your termination strategy better. Give yourself plenty of time and make sure you have the correct equipment setup or you'll have a nightmare situation. Cover crops are great but they do add another layer of management that needs to be taken into consideration.