mairomaster , 05-03-2017, 01:39 AM
I don't think it matters at all with small PCBs with symmetrical stackup. In theory you might get the board bent because of the thermal expansion during manufacturing. That normally happens if you have difference in the copper density between the two halves of the stackup. For example you have planes on layer 1 and 2 and you have tracks only on 3 and 4. However with you case it is symmetrical so it's OK. Also your board is very small, the problem is more common with big board, logically they can bend more easily. Apart from that, the problem in general has been much more often in the past than it's nowadays.
Sometimes the manufacturers recommends using fills so they don't need to etch off too much copper. Long etch times might be a problem with small details (thin tracks, fine differential pairs) than need a good control of the etch time to be produced accurately. I can't see this being a problem with your board as well.
You can either decide to go with their advice and be absolutely safe (takes some effort to create and maintain the fills) or just tell them that it is fine like that. If they can really have problem with such a simple board, they would be quite a bad manufacturer.