Using one overhand and one underhand grip allows you to pull more weight. But the underhand grip also has the nasty tendency to overload the long head of the biceps in a vulnerable forward-shoulder position that's been the cause of many powerlifting injuries.
If your purpose for deadlifting is to pull a maximal weight in competition, go for the mixed grip. You've likely examined the risk-reward ratio and know what you're dealing with. But for most, the benefits of the mixed grip don't outweigh the risks.
Instead, use the double overhand grip. Training in a more symmetrical position and allowing the shoulders to stabilize and centrate under loading is transferable to nearly every lift and physical activity.
Though this grip may be more limiting in terms of loading abilities than its mixed counterpart, the strength gain will be more transferable and functional for building resilience and preventing not only biceps injuries, but injuries across the board.
Depending on your stance preference, either sumo or traditional, you'll place your hands about shoulder-width apart. To know where you should be, drop your arms down to your sides and extend your thumbs on each hand touching the sides of your legs. There is your starting position. This is approximate, so feel free to move your grip in and out a half inch at a time to find your perfect overhand grip width.