With balanced shape and 15-17 points (or whatever else your 1NT range may be) should you open 1NT even with a five-card major? Some people never do. Some people always do. Some people sometimes do. I've never read a completely satisfying case for any side of this argument. Here's a suggestion based on the law of total tricks.
If you open 1NT with a five-card major, you may miss a 5-3 trump fit. You may be able to back into it on an auction like 1NT - 2NT - 3 - 4 if partner recognizes that 3 offers him a choice of 4 or 3NT. But in other cases, such as 1NT - Pass or 1NT - 3NT, you will be playing in notrump no matter what.
On the other hand, if you open 1 or 1 with 15-17 points and no second suit, you may have difficulty communicating your strength with your second bid. For example, if the auction starts 1 - 1, rebidding 1NT is too weak, while rebidding 2NT is too strong (and either 2 or 3 is misleading about both strength and length). Some people say they underbid with 15 points, overbid with 17, and open 1NT with 16, but that strikes me as a very unsatisfactory solution.
Especially at IMPs or rubber bridge, it may be safer to play a trump contract rather than notrump when you have a 5-3 fit. If the trump contract doesn't produce at least one extra trick, however, 1NT may make when 2 or 2 goes down one, or 2NT may make when 3 or 3 goes down one, or 3NT may make when 4 or 4 goes down one. At matchpoints, it is certainly better to make an overtrick in 1NT than to make 2 or 2 exactly, or to make an overtrick in 3NT rather than making 4 or 4 exactly. But none of these desirable outcomes will happen if the trump contract produces more tricks.
When does a trump contract produce an extra trick? The law of total tricks seems to answer this question. In applying the law to a notrump contract, one is supposed to count seven "trumps". When the alternative of a 5-3 fit is available, for eight trumps, we should therefore expect one trick more in the trump contract. If there are positive adjustments to be made in the law, such as "pure" high-card values, the eight trumps may even count for nine, suggesting two tricks more in the trump contract. But if there are negative adjustments, such as "soft" holdings in the side suits, the eight trumps may only count for seven, and a notrump contract may produce the same number of tricks.
So my idea is that soft values in the side suits, or other factors that require negative adjustments in the law of total tricks, should suggest opening 1NT rather than 1 or 1, especially at matchpoints. I haven't really tested this yet. I'd be interested to hear from anyone who has.