I think that's pretty much an urban myth, searchlight, about why shops charge £9.99 etc.
The chances of buying an exact number of litres is very slim. What if you got 1.01 litres instead or 0.994 litres.
In practice they won't sell you an amount of only 1 litre. If you bought say 10 litres at 119.9p a litre it would result in a bill of £11.99 so there is no rounding issue.
Of course if you bought for example exactly 3.000 litres it would be interesting to see what they charge- I guess they wouldn't mind rounding down a part penny. In practice the machines perhaps work in full pence anyway.