Is it okay to salt beef before or while cooking it?
Several sources as Cook's Illustrated, Alton Brown, and Anita Lo practically insist that you salt steaks before cooking them.  I don't think McGee experiments with or discusses exactly when to season/salt a steak in his books, but he has reportedly stated that he is also in favour of pre-salting.  The fact that so many people seem to prefer this technique would seem to indicate that there's at least some merit to it, even if the science isn't well-understood.

All of the arguments I've heard against salting steak (or beef in general) before cooking seem to be anecdotal in nature.  When pressed for an explanation, many of these people claim that the salt draws out moisture which will subsequently dry out the cut.

In practice, the amount of moisture it draws out is practically negligible unless you actually cure it, which means using lots of salt and letting it sit that way for a long time.  I don't know anybody that does this.  Well, hardly anybody (warning: do not follow the advice on that page unless you are fully prepared to ruin a perfectly good steak).

When pressed, most of these people (including Anita Lo, above) say that the water it draws out to the surface will inhibit the Maillard Reaction.  This is true - the presence of water does inhibit the Maillard reaction and any significant quantity of water will give you a steamed gray steak instead of a delicious seared brown one.  But the key word here is significant.  No reasonable amount of seasoning will draw out so much moisture that you actually end up with a puddle of boiling water underneath the steak, and even if it did, you would simply pat the steak dry before searing it.  You do pat your steaks dry, don't you?

Also note that this applies to dry seasoning.  When marinating a cut of beef it is another story entirely.  Salt in a marinade really does create brining and that will tend to make the meat juicier.  When meat brines, it absorbs extra moisture - the meat still loses moisture when cooked but the added moisture from the brine helps to offset it (this, again, is all in McGee).  A saline solution also dissolves a portion of the tougher proteins in the meat, resulting in a more tender result.  Salt is a great thing to have in a marinade, which is why some of the simplest of marinades - soy or teriyaki sauce - are so effective.  Just, again, make sure to pat that beef dry before pan-searing it if you want to get any sort of browning going on.

Anecdotally - for what little that's worth - I find that there's very little difference in tenderness whether seasoning with salt briefly before or shortly after cooking (before resting).  I've done both and I honestly don't think I could identify which was which in a blind test, except perhaps for the consistency of the "crust" that forms when you really pile on the salt - this is a favourable result for many, and I'll often do this if I'm in the mood.

But all in all, the scientific data on this subject is scant; the results are very inconclusive.  And it just doesn't matter that much; the debaters seem to put entirely too much emphasis on this point when there are far more important factors in preparing a great steak or roast, such as additional seasonings, what it's seared in, temperature and heat distribution of the pan, and let's not forget the cut and grade of meat.  These things have profound effects on the final result, and if I were obsessive about beef to a fault, then I would concentrate my efforts more on finding better quality ingredients and equipment rather than fussing over the salt issue.