If you're running a business (even a one-person business, and I recall we've already remarked on the large number of these in Greece), you usually have a VAT account with the tax authorities. In your books, you have an account for VAT collected (paid to you by clients when paying bills) and another for VAT expended (on your investments and expenses).

At the end of the year (or other accounting period according to the type and size of business) the tax authorities may owe you a refund (if you have paid out more VAT than you have received), or, (more probably), you owe them the difference between the outgoing and the incoming.

Of course, just as you need invoices to back up your incoming VAT account, you need receipts for your expenses (to prove what VAT you've paid out). The taxi journeys would need to be necessary to your business, of course. Forgetting to get receipts would just automatically decrease your VAT expended account, with the result that you would pay more VAT than necessary.