Who should complete a tax return?

http://www.hmrc.gov.uk/sa/need-tax-return.htm

The above says that you need to submit a tax return if you are a company director or have income of £10k or more from savings and investments. There's other conditions but these are the conditions that affect most of my company director clients.

If you have submitted a tax return previously and HMRC say that they don't want a tax return in future do you only have to submit a tax return if there's a new source of income (what if that source doesn't result in extra tax?) or the current sources of income mean you have tax payable? I suppose I should still review the position in case there is a refund of tax.