If you weigh yourself after relieving your bowels, your weight will be slightly more accurate as far as what YOU weigh, without as much of what you ate. Weighing yourself at the same time every day is probably best. During a typical day, most people follow a pretty well set routine. We eat at about the same time every day, drink about the same amount of liquids at about the same times. So, as long as you're weighing in at about the same time every day, fluctuations in your weight will be more accurately depicted.
My personal preference is to weigh myself at night before going to bed, but many people prefer to weight themselves in the morning. In my personal experience (having weighed myself at night and then the following morning) the difference in weight between night and 8 hours of sleep and a bathroom break later is rarely more than a pound.
Additionally, your weight fluctuates up to around 5 pounds a day anyway, due to all kinds of reasons. A person should only consider their weight over a longer period of time, such as a week or even a month to make a truthful analysis of whether they are losing or gaining weight.