If an ability comes up that requires a target and there are legal targets available, you must use that ability even if you may not want to. However, the effects are all good for you or bad for your opponent. In some cases, you may not be able to take advantage. For example, if it ends up that creatures you control get +1/+1 until end of turn but you don't control any creatures, you're out of luck. But nothing bad will happen.