Some stores, like Nordstrom and Ron Herman, offer complimentary alterations when you buy jeans from their store.
As for what rises make you legs look longer, I think it also depends on your proportions. For example, you can be short but have long legs in proportion to your body. Alternatively, you can be tall but have a long torso, making your legs short in proportion to your height. A general rule of thumb would be that the higher the rise, the longer it will make your legs appear.