God, I hate having to make these big decisions.
We put off having DS2's MMR until I was absolutely sure there was no way he was on the autistic spectrum (DS1 is, although I don't blame the MMR itself for that).
I am not at all sure about DS3, but after much fretting we've decided to go ahead and get it done now because there's lots of measles about in the area.
So, fingers crossed, I suppose...