in my opinion the best and easiest thing to do is to set daily maximum amounts. This will allow you to stay within your target budget range, without having to pause and unpause your campaigns on a daily basis. ADwords makes this easy.
Also, if you are really concerned about your budget, and maximizing quality clicks, I would strongly consider putting [brackets]around your keywords, and opting out of content ads.
Lastly, consistency pays. Unless you know specific behavioral characteristics about your audience, i.e. purchasing specific times of day, or specific time zones, et al., it pays to be consistant with PPC buys on the engines. Some days will be better than other, so to will some weeks and months. ROI is best looked at on a quarterly basis IMHO.
good luck.