I usually pick a reasonable pronunciation based on the most similar real language, but in cases where the pronunciation is completely unclear or ambiguous, it tends to nag at me. The pronunciation guides some fantasy authors include can be very helpful when that happens.
Of course, if the author's pronunciation isn't reasonable, it's even more irritating. A while ago I read a fantasy in which the author wanted "kitsune" to be pronounced "kit-SOON-ay". I spent the whole book wondering if she also collected "net-SOOK-ay".
