In the U.S., we have a pay-as-you-go tax system. That is why we have withholding for employment income and estimated taxes for all else. But paying estimated taxes is only required to the extent that there is a financial penalty for not doing so. Some people automatically accept the penalty rather than pay estimated taxes. Others try to game the system, file appeals, etc. People find strange ways to amuse themselves.
If your income fluctuates, you're probably better off paying estimated taxes, even if it is inconvenient. If you have a job, you're probably better off claiming lots of exemptions (typically 14) to reduce your withholding, and then paying enough estimated taxes to get safe harbor.
The easiest way to estimate taxes is to pay 100% of your 2007 tax liability, one-fourth each quarter. You can estimate any amount, but if you come up short, you have to pay a penalty (unless you qualify for safe harbor). If you estimate too much, you are making an interest-free loan to the government.
[rwk]