Helen,
Whether or not in practice you actuallystrum in-out or out-in when playing a tune should depend on the tune and how you want to play it. However, it is very important that you have a steady strum and stay on beat. It might be a good idea to practice with a metronome for that reason. Once you have mastered steady playing, you can vary it according to the needs of the song. But first you have to master the ability to play steadily.
There are innumerable advantages to developing a steady in-out or out-in strum. I outline how to develop that kind of a strum in a series of three instructional videos (amateur ones, of course) which I made available here:
The strum I teach in that series of videos is basically an eighth note pattern in which you strum out on the down beats and in on the up beats. So when you count 1&2&3&4& you strum out on the numbers and in on the &s. You can reverse that and strum in-out instead, but what matters is keeping it consistent. Once you have a machine-like regularity, you can begin to accent or stress certain strums and "swing" the rhythm, too. That is where the first video ends. Once you have that accented out-in strum down and are swinging it, you have a rhythm that you can use for any tune. And what is great about it is that you don't have to count beats; the strum does it for you. If you accent the first beat, for example, you will always know where you are in a song. And you know to strum quarter notes, half-notes, and the first half of eighth note pairs on the out strum and the second half of eighth note pairs on the in strum.
The second video then gets more creative with rhythms, explaining how you can skip strums, mute strums, and accent strums to get really complex rhythms. In the third video I move from strumming across all the strings to adding single notes, for there is no rule that says you have to strum all the strings on any beat. You can play one, two, three, or no strings as you wish. So that technique for strumming also allows you to begin flatpicking.
Anyway, feel free to take a look at those videos and ask me any questions you want. I will never tell anyone that they have to play a certain way. But I can tell you that I find it helpful to develop a very steady back-and-forth right hand.
At the local dulcimer group I started last fall, there are basically two of us who are the leaders since we are the most experienced. Ron Beardslee does not strum in the method I do. He is very precise and only plays exactly the notes in the music or tablature. He is able to get very clean and sparse arrangements that sound great. But he counts in his head all the time, and he has difficulty just playing along with people and following other musicians rather than playing from music. I have other skills and other problems. When I play my right hand is constantly moving out and in. I don't always pluck a string, but more often than not I do. I often stick in chords or bass notes or an arpeggio or occasionally a little lick when it does not appear on the tablature. Ron calls this me being fancy, but to me it is just playing. My hand is moving all the time. And because the rhythm of my hand does the counting for me, my brain is free to watch other musicians or think of something to add or whatever. I don't have to count to get that 3& beat since I know I'll catch it on the in strum after the accented 3 beat of the measure.
Again, there are a lot of ways to play the dulcimer. You do not have to develop a steady out-in or in-out strum. But you should aim at developing a steady strum, and to do that you either have to count constantly in your head or you have to develop that strum you are describing.
One final point: Ken mentions ballads and the desire to keep the rhythm of the melody. That is indeed one very good way of playing. But there are others. Jean Ritchie, for example, does a lot of counter-melody work. Other modern players who also sing, like Sarah Morgan, develop a rhythmic strumming that accompanies but does not copy the singing. But I'd like to make a different point.What do you do if you are playing a slow ballad in which a single note carries on for a whole measure or two whole measures? Do you strum once and then count in your head (1&2&3&4& 1&2&3&4&) before playing another note? OK, you might do that on occasion, but if your song has a lot of long, drawn-out whole notes, you need to come up with something else. A regular, swing-version of an in-out strum all by itself can help you fill in those spaces in a way that does not detract from the melody. That is something I demonstrate at the end of the third of those instructional videos.
I won't be insulted, Helen, if you don't find my videos helpful. Don't feel obliged to watch them. But I do think they are a good introduction to the benefits of developing a steady out-in right hand. So if you are asking why you might want to learn to play like this, the videos try to answer that question. And of course, contact me with questions or comments or points of disagreement or whatever.