The only reason most people take dextrose is to give them a pwo spike to help muscle growth and recovery. So no you don't need to be taking any on your off days. I'm assuming you're taking creatine monohydrate? If so, it doesnt really matter how much water you take in at the time of consuming the creatine, but make sure to drink a gallon of water throughout the day. Many people take creatine both before and after, it's really a matter of personal preference. You also don't need to be taking any dextrose pre wo, just take in some protein ( a must) and carbs from fruits or something if you feel you need the energy boost. The best pwo carb mix is claimed to be a 50/50 ratio of dextrose and maltodextrin, and you should take anywhere from 50-100 grams of carbs from these two sources pwo depending on your body weight.