How do you decide whether or not to reinvest dividends?
If your goal is growth, then my recommendation would be to reinvest all dividends. You're going to need some cash flow as time goes by, but the more dividends you reinvest, the more quickly your stock holdings will grow in value. This will help you reach your goals faster than if you were just sitting on cash and not investing it at all.
If your goal is income, then my recommendation would be to only reinvest half of your dividends. This will give you some cash flow while still letting your stock holdings grow in value over time—which means that when it comes time for retirement, you'll have more money than if you had never invested any of those dividends.