If you don't have a lot of room (i.e. you know you will be filling near the rim); then adding water to grain means you will have all your grain in. You can always hold back water, your mash will just be less dilute. Palmer tends toward thinner mashes, where dry spots are not as easy to get. If you are doing thick mashes (< 1.25 qts/lb), then adding grain to water would probably be better. But if you have plenty of room (helps with stirring too), and aren't too thick, you could do either really. Assuming Palmer doesn't know something Miller or Snyder don't.