No, you shouldn't take Viagra every day. The effects last only for a few hours. It is best to take Viagra about an hour before you plan on having sex. Taking it once a day wouldn't get you the maximum effect, and you risk unnecessary side effects.
Some studies have been done on people taking low doses of Viagra on a daily basis. This might be effective, but unless if your doctor has specifically recommended this, we don't think it's the best way to take the drug. Timing it right before the action will get you the best effect with the fewest side effects.