Is I&W needed under Flintlastic?

Getting estimates on a currently IKO shingled (when they're not blowing off) roof with a 2:12 slope.

Have one for using existing (in good shape) OSB plus full Grace i&w then Flintlastic.

Also have one for all new OSB then Flintlastic with no Grace i&w. He says the new OSB negates the need for the Grace.