Food combining (food combinations) appear within many nutritional guidelines. Of course, as with fitness, if you look around enough, you will find one that disputes another. As a former Precision Nutrition(ist), living with a raw foodist, I have insights towards a variety of food combinations that I humbly choose not to apply.
Of course, each of the aforementioned approaches work, for many people. However, using myself as the guinea pig, I tend to utilize and endorse what works (worked) for me. Over time, I've found food combinations to be very important in relation to my own nutritional, fitness, and body composition goals. Admittedly, I'm somewhat jealous, and (of course) a bit skeptical, of those that do not have to watch what they eat so carefully.
The guidelines I follow, utilizing Ori Hofmekler's Warrior Diet are:
a. Vegetables and protein combine with everything
b. Do not combine sugar with alcohol
c. Do not combine grain with alcohol
d. Do not combine grains with sugar
e. Do not combine nuts with sugar
f. Do not combine nuts with grains
In my own experience, a great deal my own misunderstanding about food combinations arose from erroneously categorizing foods. For example, using the Warrior Diet vernacular, fats, such as olive oil or EFAs (which I include in every meal) are separate from fat fuels such as nuts or seeds. Similarly, the carbs that naturally appear in vegetables (which are also included in each meal) are distinct from the carb fuels found in grains and certain vegetables (such as sweet potatoes).
The goals of this particular nutritional approach deals more with hormones and detoxification than caloric recommendations. For this reason, I've found, the guidelines to be essential for its success. Of course, utilizing real food, eating your vegetables (lots of them), and cutting out the crap will definitely point you in the right direction. However, it may be best to find what works best for you, and apply it liberally.
Best of luck with your training and nutritional goals.