Most pro airsofters will say never to reuse bbs. This is true in most cases to most guns. A bb with a little dent will cause damage, especially to high quality guns. However, with the proper target, you will be able to save money and not risk breaking your gun.
First, you need to build a good target. Do NOT use any sort of hard material, or you will risk denting the bbs. A basic target consists of a cardboard box (the thicker, the better), paper, and something soft that will fit in the box (ex. small pillow, sweater, folded up blanket, etc.). Start off by tracing the outline of the paper on the biggest side of the box with a pen, then cut it out at least 1/2 an inch inside the line, so you have a border to attach your paper to. Now insert the soft object inside of the box so it is against the opposite side of where you made the cut. This will cushion the bbs, and will not harm them in any way. Last, get some tape (do NOT use metal fasteners, the bbs will rickoshet and dent/break) and tape each side of the paper to the box so the paper covers the hole. When you shoot the target, the bb will only have to go through the paper, then will be stopped by the soft object inside the box. Because they do not hit any hard surface, they will not be damaged and are ok to reuse.
Next, you must inspect the bbs after you collect the from your target trap. Unless you use a jacket with metal zippers, you shouldn't have any problems, however it's best to be safe than sorry. If you see any obvious signs of damage, such as dents, chips, and cracks, throw it away. If it looks fine, it should be safe to reuse.
If you are in the middle of a war and find yourself out of ammo and don't see anyone you can borrow bbs from, for the sake of your gun, don't reuse bbs on the ground. The reason is, they have probably hit something hard and have dents in them. Also, even if they haven't hit anything hard, they are on the ground and are full of dirt, sand, dust, anything like that, which also cause damage to your airsoft gun. You're better off looking for a teammate to borrow bbs from, even if it risks getting shot.
If you do own an AEG, do not reuse bbs at all. Period. It is not worth risking an expensive gun. Reusing bbs is ok for spring airsoft guns, as long as you are careful of the conditions of the bbs you put in your gun. The pros of reusing bbs are that you don't have to keep buying bbs, but the cons are they could ruin your gun.