One of the most important things you can do to get all that you can out of your PPC campaigns is to practice granular ad group organization. This does not mean you leave out 1-2 word phrases because we know they carry a lot of needed volume and conversions. So, what do we do with them? Put them in their own campaigns where you can better control them. Then, pull 3-word phrases that perform well enough to have their own ad groups and create them as you optimize your account.
If you are spending your whole PPC budget on what works, you are falling behind. Almost every author or speaker on the subject will tell you that as soon as they are ready to publish their book or deliver their presentation, the information has changed. This industry, like most of the world, is moving fast.
If it is an appropriate time of the day for your ads to show, you never want them to be shut off because your account has hit its budget. If this is happening, you are not spending your money as wisely as you could be.