Each brand/model battery has a recommended max charge rate (0.5C or 1C, for example) and anything under that is considered safe to charge them at by the manufacturer
However, using faster charge rates does put more strain on the battery, the same way that discharging them at higher rates does, and will shorten the overall lifespan of the battery. Scott makes a good point with "lowest is best" and by charging at lower rates you theoretically will be prolonging the overall life of your
batteries. Yes, it takes longer to charge them, but for some the benefits are worth the extra time