One of the obvious questions in a sport with bodyweight categories is which category to compete in. This will vary among athletes and sports. The scheduling of weigh-ins is probably the biggest determining factor in how dramatically an athlete can play with his or her weight. In sports with weigh-ins 24 hours before competition, huge amounts of weight can be lost through dehydration to make weight, and then subsequently gained back with no detrimental effects. In the sport of weightlifting, on the other hand, weigh-ins are from 2 to 1 hour before the lifter's session. This greatly reduces the amount of weight that can be lost because there is insufficient time to rehydrate adequately. This doesn't mean weight loss through dehydration can't be done, it just means it will either be a smaller amount of weight or the effects will cut into performance.
So, the question remains: What should your bodyweight be? For most athletes, the choice will be made based on proximity to natural bodyweight. For example, if an athlete walks around normally at 83 kg, he is going to be an 85 kg lifter. In a situation in which natural weight is right in the middle of two classes, the choice should be made based on a couple factors. First is what's possible and/or practical for the athlete. If he is 81 kg, he's equidistant from the 77 and 85 classes, so in theory he could go either way. But if this athlete is a bit soft at 81 kg, it makes sense to try to lean him out to 77. If instead he's already pretty lean and muscular, it makes more sense to fill out to a full 85. Next to consider is simply the athlete's ability to gain or lose weight. One will always be easier than the other for an athlete. If an athlete really struggles to gain weight, it may not be worth the effort, and this is even more true for losing weight, since it's obviously more taxing and has the potential to be quite detrimental to strength and recovery capacity. Third, if the athlete is at higher competitive level, his/her competition needs to be considered. Based on his abilities, our example lifter may be considerably more competitive in the 85 kg class than the 77 kg class or vice versa, which case, if it's practical, it's wise to move up.
Along these same lines, in some cases a lifter will choose to move up significantly in weight to be competitive. This will usually be the result of that lifter being relatively slender for his or her height. In reality, bodyweight classes for lifting are arguably more about regulating height than weight. Smaller lifters have better leverage and are able to lift more weight relative to their bodyweight. This is hugely problematic for tall, thin lifters who would be competing against much shorter, stouter lifters. There are certainly world class lifters who are surprisingly slender, but they are not normal. In order to ensure more reasonable competition, these lifters will usually need to increase their bodyweight fit into a category that places them up against lifters of similar stature.
The next question is, Do you need to fill out the full weight of the category? Again, the answer depends a bit on the athlete and the situation. For the recreational lifter who is content doing local meets and isn't concerned with moving up in the ranks, bodyweight becomes largely irrelevant. This individual will simply compete in the class into which they happen to fall naturally. I know of quite a few athletes who have lifted in heavier classes being as little as a single kilo over the lower classes cut-off. There is nothing wrong with this if it suits the athlete's goals.
In the case of a more competitive lifter, it's important to maximize bodyweight within the allowable limit. While not necessarily true in other sports, in weightlifting, there is no benefit from being lighter than necessary. More bodyweight, even if less than perfect in composition, will bring with it more muscle, which is more potential for strength. Composition can always be improved over the long term while maintaining essentially the same weight.
Finally, at what weight should an athlete train? Some lifters prefer to spend most of their time heavier than their class and then drop the necessary weight as a meet approaches. This may be because their natural weight is a bit heavier than their class, or it may be because they feel they can train heavier and harder at the higher weight and this is beneficial both physically and psychologically, and there is minimal to no loss of performance when the weight is lost. I suggest that people restrict this to 2-4% of their competitive weight to prevent too much difficulty dropping weight and limit any false sense of ability that disappears when the weight is lost.
How to actually cut weight
is an entirely different topic that I will cover another time. For now, just keep in mind that the more you have to cut and the shorter the period of time in which you do it, the more taxing and harmful to your performance it will be.