It would be better to read The Last Wish first, however its not necessary. I read the Witcher Saga before the short stories collections and while it felt sometimes confusing as I didn't know certain characters and their histories and relations, it was usually explained fairly well in the saga. So I think you should be ok reading Blood of Elves before The Last Wish.

Anyway, it will take several years untill the whole saga is published in English, I believe the second installment is due to be issued this Autumn, so you can get the short stories before the series ending.

There is however another problem connected to this because the publisher decided that only the first (The Last Wish) of two collections of short stories will be translated into English before the saga. And it is the second collection of short stories that directly precedes and introduces the saga, so it will still be a little bit confusing even if you have read the Last Wish first.

And finally, I should probably warn you that the first volume of the Saga (Blood of Elves) is a relatively slow book. It stops more or less when the action starts and you should probably read the second book (Times of Anger) to get the real feel of the Saga.