Dr. Danny is the man to give you a complete answer to your question, but I thought I'd at least say just this:
It's tough to say if it is "common" for men to get headaches as a natural and expected side effect of a certain dose of Viagra.
Everyone reacts differently and most men can probably expect that they will NOT experience any or all the possible side effects. These side effects are published because they MUST be: a patient needs to know what is POSSIBLE when taking ANY medication. But that doesn't mean it is PROBABLE.
In fact, the literature which should have been included with your Viagra should give you a detailed list concerning the frequency of side effects in a certain sample size of men.
Here's some basic info from Pfizer:
http://www.viagra.com/consumer/pi/prodInfo.html
I did not notice a chart or table of incidence of reported side effects here, though.
If you experience headaches taking a certain dose, tell your doctor. He will probably recommend a reduced dosage. This can be done over the telephone.