I auto re-invested for well over 20 years. It was on autopilot as Chad stated, free and because I paid commissions back then a small purchase made little sense. Now days my dividends are very substantial money and commissions almost free. I consider myself a value investor so I collect them up for a month or so and I decide how the funds are re-invested. My best personal example is I have a lot of utility shares. They pay big Divs and are very clearly overvalued by any rational measure. Knowingly buying shares of stock I am certain are way over historical valuation range would violate my strategy. If I wouldn't make an outright purchase at current price, then why would I DRIP? It's one in the same. You are buying over priced shares with YOUR money.
So my answer is "it depends". If it's big bucks and it actually matters, then maybe put the decision where I think it truly belongs... in your hands. Are you making frequent share purchases each month anyway? If so, that nullifies the commission avoidance excuse. That said, you won't go broke doing it the easy way and just pay too much for new DRP shares. If I owned 50+ stocks and couldn't possibly track them as well as I know I should, well then I might just close my eyes, DRP, and hope Mister Market takes care of it for me in the end.
So my answer is "it depends". If it's big bucks and it actually matters, then maybe put the decision where I think it truly belongs... in your hands. Are you making frequent share purchases each month anyway? If so, that nullifies the commission avoidance excuse. That said, you won't go broke doing it the easy way and just pay too much for new DRP shares. If I owned 50+ stocks and couldn't possibly track them as well as I know I should, well then I might just close my eyes, DRP, and hope Mister Market takes care of it for me in the end.