Doesn't this depend on whether they are using a Weirdboy-navigated ship, or they are just hitching a ride on a Space Hulk?? According to Navis Primer, Weirdboys can Navigate the warp, via instinct!
Oohhh, found a great quote from Navis Primer..
"Dere's dis fing, see, what comes from all Orks. It goes round us an' through us, and keeps da kosmos togevva. It's called da Waaagh!, an' it makes Orks Orky. It gets bigga when we does, and gets stronger when we's fightin' an' winnin'. An' when dere's enuff of it, lots and lots of it, see, den speshul fings 'appen. Like really 'uge wars!
-Ogbad, Ork Warphead"