On a 2.5 to 3 metre span it shouldn't make much difference, especially if your joist spacing is 16in (or just over 400mm). In your case herring bone strutting is clearly visible, so that shouldn't mske any appreviable difference.
Whilst it is true that some manufacturers permit this within specific parameters (such as the maximum distance from a joist being, say 100mm) you would need to check if the manufacturer of your boards permitted this and under what circumstances. In general I think most responsible tradesmen would take the belt and braces approach of adding extra support beneath end joints that don't otherwise land on a joist. You just never know when somebody is going to roll a piano (or any other heavy load) across that unsupported joint. Being risk averse, and not liking to do call backs (for which I wouldn't get paid) I tend to just do the small amount of extra work it takes and then sleep soundly at night!
Do it as you go, yes, but I'd probably put the noggins in 100 to 150mm in from the edges. That way you don't exceed 400mm span without support
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