Having the sums done for you is not the only advantage in getting your return in promptly. Employees who have underpaid tax by up to pounds 1,000 can have that tax collected through their PAYE code for the next complete tax year as long as they meet the September deadline. If not, the full amount has to be paid at the end of next January.
Even if you do not manage to get your return in by the end of this month, the Revenue says it will still try to work out how much tax you owe or are owed. But they cannot guarantee they will do this before 31 January - the final date for tax returns to be in and any tax owing to be paid. Failure to meet that deadline means an automatic pounds 100 fine.
So if you miss this month's deadline and do not intend to do your own calculations, try to overestimate the tax you owe and pay that amount by the end of January. Any overpayment will be reimbursed.
So far, 2.8 million of the 8.9 million people who were sent a self-assessment tax return have completed them - slightly more than at this stage last year, says Jane Ashton of the Inland Revenue.
If you are happy to calculate your own tax, you have some breathing space. And, according to the Revenue, doing it yourself is not too difficult. "You copy figures from the tax return on to the tax calculation guide and add them up," says Ashton. "I would say it is very simple."
During the week, you can get help over the phone from your local tax office, whose number is on the front page of your tax return. Outside office hours there is also a helpline for any self-assessment queries.
Is it really important to meet the September deadline? It is, unless you want to do the sums yourself, says Steve Midwinter, partner at the accountants, Deloitte & Touche: "I think calculating your own tax is very difficult for a normal person. The tax affairs of anyone obliged to complete a self-assessment return will be more complicated than average."
The best way to find a good tax adviser is through personal recommendation. Failing that, call one of the professional bodies. How much they charge varies enormously. For advice as well as completing a return you could expect fees of at least pounds 100 to pounds 200, while those with assets could get a bill for thousands of pounds.
Inland Revenue helpline: 0645 000444; The Institute of Chartered Accountants in England & Wales: 0171-920 8682; The Chartered Institute of Taxation: 0171-235 9381; The Tax Team: 0800 393520; Deloitte & Touche: 0171-936 3000