Hey Andy - Welcome to the group. Happy to have you here. Great question, yes you are correct that common practice is to have the tongue on the left side of the hole you are playing, and also fairly common practice is to block 3 holes to the left of that hole.
So to play hole 4, tongue blocks 123
To play hole 5, tongue blocks 234
To play hole 6, tongue blocks 345
Some people might make there mouth narrower and block fewer holes, but I recommend practicing it like this, and for the exact reason that @slim mentioned is you can pull your tongue off, and all of a sudden you have a chord below the note you are playing.
Our ear gravitates toward the highest note of a chord, so having the note we want to be perceived as the melody on the right side of the chord on the harmonica is very helpful. This is the sound that we love so much from Little Walter and other masters of this style.
So if you’re interested in getting that Chicago Blues kinda rhythmic style, I agree with wisdom of spending a lot of time practicing with the tongue always to the left. (Of course, if you really want to tongue block hole 1, then you have to block on the right - it’s you’re only option! lol.)
Now, having said that, Joe Filisko taught me that one of the great benefits of tongue blocking is you now have “2 mouths” (the left side and right side of the tongue.)
This is really nice if you’re wanting to do some big interval jumps, you can play the low note out the left side of your mouth (with tongue blocking notes on the right) and then with very little movement switch to playing out of the right side of your mouth (with tongue blocking notes on the left.)
This is seen easily when doing the technique known as Split Shakes. One of the coolest split shakes in the blues idiom is the -35. It’s a cool variation on the -45 shake. I think the -45 shake, or trill as I like to call them, is probably the most iconic sound a harmonica can make. But the -35 is a super-hip variation on it. On the -35 shake, your tongue is blocking the -4 and you’re shaking it real gently to trill between the -3 and -5.
BTW - I grew up playing octaves, but only got into tongue blocking in the last few years. So the only tongue blocking I’d ever done was to play split-4’s. It took some time to figure out how to narrow my tongue enough to play a split-3 (which is what you’re playing when you do a -35 shake.)
I first really got turned onto the split shake thing seeing Joe Filisko do his variations on Simple Gifts - at one point he plays the melody doing split shakes on octaves and that just really opened my mind up to the possibilities of this kind of technique, because I’d always wished I could make big interval jumps more quickly like that, but I’m still in the early stages of exploring it.
All that to say: there is a time and a place and reason for practicing with the tongue on the right side of the note, and the main reason, imo, is in this type of endeavor.
Other than that, tongue on the left side of the note is gonna serve you well not only for great rhythmic stuff, rapid chord vamping, etc., but also for slapping too.
The tongue slap is a technique where your tongue is off when you play the desired note for the tiniest fraction of a second, and then is “slaps” creating a more percussive attack on the note. This technique is analogous in my mind to a guitar player raking his pick percussively through muted strings on his way to plucking a held note. It gives it some serious attitude and umph. The reason I put “slap” in quotes here is becuase YOU DON’T HAVE TO SLAP YOU’RE TOUNGE HARD against the comb to achieve this technique. I spent a year or 2 thinking I had to whack my tongue against the harp, lol. Nope. It’s all about the timing. Making it so short is the trick, and I’m still working on this technique as well!
Well that might be way more than you ever wanted to hear in response to your question, lol. LMK whatcha think!