Here is a different viewpoint: I say that it depends mostly upon the film, itself. If you have a standard film like Delta 100 or TMY 400 it matters to be within, say, 10% of the ideal. If you are using a microfilm like Kodak ImageLink that accuracy matters even more, assuming you are aiming for continuous tone and not extreme contrast. In sum, inherently contrasty films demand even more accuracy with development.
Taking an atttitude that variable contrast paper will always save the day is faulty thinking. You should get your situation in order and be able to produce consistent results time after time. Naturally, psychfunk, if you dilute you get to have a more lengthy development time in order to achieve the same gamma (contrast relationship between the negative and the actual scene). This can mean more accurate development because it's easier to time 10 minutes than to time 3 minutes (because of the inflow and outflow of developer which does, and does NOT, 'count' as actual development time). With a 10 minute development time, the time needed to fill and empty the tank does not matter as much as with the 3 minute development time. - David Lyga