Technically your ads only cost you money when someone clicks on them. So, altering your budget in a historically slow month shouldn’t effect you. If customers aren’t on Google looking for what you sell, you won’t be charged for clicks. However, if you’re exhausting your daily budget during busier months, you may find that you still use your full budget during soft months. Thus, not decreasing your AdWords investment. Perhaps an alternative consideration would be: should you raise your budget so it’s not exhausted each day and you’re getting all the available clicks/traffic.
Because of the way Google Search advertising works, charging you only when you have a “hot prospect”, it’s really the perfect form of marketing when you have all the settings dialed-in and you’re sure you’re not wasting money on irrelevant searches. Google Display ads would be a different story because they’re used more effectively to promote your brand rather than capturing customers at the moment they’re ready to buy.
It’s wise to put as much budget as you can towards AdWords as long as you monitor your campaigns often to make sure the extra budget is generating additional traffic.