Misconception: Don't open 1NT with a 5-card major.
Truth: 1NT is a better descriptor of a balanced 15-17 point hand than opening 1-of-a-major.
Opening 1NT with a five-card major is the new normal. There are several advantages to opening 1NT with a five-card major (and 5-3-3-2 shape) . 1NT shows a very specific hand type: 15-17 points with a balanced hand. That is a lot more information than opening 1 or 1 which shows five-plus cards in that suit and 12-21 points. Not only is the point range narrower, but knowing that partner has at least two of everything means that you know eight cards in his hand rather than five.
There are other advantages: One of the first things we learn as bridge players is how to bid after partner opens 1NT. As a result, opening 1NT allows partner to a) evaluate his hand and b) use the tools at his disposal to find a major-suit fit.
For an example of when this is better If you open 1 instead of 1NT and partner holds:
Partner will pass, leaving you in a six-card fit. Meanwhile, opening 1NT leaves you in a much better 2 contract in either a seven- or eight-card fit.
Opening 1NT also avoids a frequent rebid problem. If you open 1 and partner bids 1, how can you show 16 points and a balanced hand? Rebidding 1NT shows 12-14, 2NT shows 18-19. You're stuck.
Lastly, opening 1NT will make it difficult on your opponents. It is harder for them to enter the auction than after 1 or 1. Also, making an opening lead can be a guess. It's conventional bridge wisdom that on the auction 1NT-3NT, you try to lead a major if at all possible. With that in mind, sometimes your opponents will lead right into your five-card major.
One last caveat. While I encourage you to open 1NT with a five-card major and 5-3-3-2 shape, I am not a fan of opening 1NT with 5-4-2-2 shape with a five-card major AND a four-card suit somewhere. With those hand shapes, you don't have a rebid problem.