is early tax payment voluntary?

If the estimated payments aren't made, there will be penalties unless you meet one of the exceptions. If what you DID pay in this year thru withholding or estimted payments is at least 90% of your total tax liability for 2010, or 100% of your 2009 tax, or you end up owing under $1000, there won'd be a penalty, just a little extra paperwork. Otherwise yes there's a penalty.

No, they are not voluntary if you have not made alternate plans for paying in what you owe. Income taxes are due when the income is earned, not on the filing deadline.

If you owe more than $!,000 at filing time you are subject to penalties for underpayment of tax. There are 2 additional safe-harbor exceptions to this rule. If you paid in at least 90% of your total tax liability for the year you are exempt from penalties regardless of how much you owe. If you paid in at least 100% of the PRIOR year tax liability, you are exempt from penalties regardless of how much you owe.

There can be underwithholding penalties if you owe over $1000 when you file your return.

Talk to your accountant.

Why aren't you asking your accountant? Yes, there are penalties.

of course there are penalties

It is called "estimated" tax.

It is not voluntary.

If you wait, there are penalties.