If you are looking to maximize performance, the biggest part of your nutrition profile that you should look at is what percentage of carbohydrates, protein, and fat you are taking in. Those percentages may change slightly throughout the year, depending on how intense your training is, but as a general rule you want to make sure that you are taking in approximately 65 percent carbohydrate, 15 percent protein, and 20 percent fat. The harder you are training, the more important it is that you are taking in adequate carbohydrate to provide your body with the necessary fuel it needs to complete that level of work. It is for this reason that low-carbohydrate diets are not recommended for very active people.
As far as calorie counting goes, if you are not looking to gain or lose weight, then as long as your weight stays stable you probably don't need to count calories. However, if you are trying to make a change in your weight, you will need to pay attention to calories. The general rule is that if you are trying to lose weight, use a few more calories than you take in. If you are trying to gain weight, eat more calories than you expend every day. Remember that caloric expenditure is reliant on two main ideas: your resting metabolic rate (what your body burns just to get through the day), and your exercise expenditure (including not only any training you might do, but also walking the dog, chasing the kids, etc.).
To calculate your resting metabolic rate, you can go to www.trainright.com/outside and scroll down to the "Calculators" button on the lower left hand side of your screen.