Orthotics in running or cycling shoes can be very beneficial for athletes, but be careful about the reason you're getting them. You can get a basic custom orthotic that supports your foot in a neutral position, or you can get orthotics that are designed to change your foot positionpresumably to improve your performance. The consideration I always want people to make is whether there's a real problem that needs to be solved.
The leg-length difference is, for instance, a tricky situation. Number one, it is sometimes difficult to discern between a true leg-length discrepancy and one that is caused by rotation around joints or overly tight or loose ligaments and tendons. Even more important, if your legs are truly different lengths, they've been like
Take the case of a cyclist who decides that after 15 years on the bike, he's going to get orthotics to take care of an eight-millimeter leg length difference. Before making changes to his riding position, seat height, and cleat position, he had no pain. Within a month afterward, his back hurt and he had sore knees for the first time in his life. Running and riding are highly repetitive activities, and for experienced athletes, a slightly "wrong" position or running stride may actually be better for long-term, pain-free performance.
Of course, for anyone who is experiencing pain, orthotics, physical therapy, bike fit, and other modalities are very effective for finding solutions that enable you to continue doing what you love. I have a severe leg-length difference as the result of breaking my femur, and I have orthotics in all my shoes. So, the long and short of it is that orthotics can solve problems or create them, so it's important to carefully consider your motives before buying. Also, keep in mind, that the orthotic is only part of the equation. If you choose to go with orthotics, make sure you're paying close attention to flexibility and range of motion so you can adapt to the changes caused by the new insoles.