If you get a chance, google Pasteur flask. That is essentially what you re making with a blowoff tube. Going in the air is ok, under water is better. With the right size tube it's no big deal either way. I have hundreds of gallons under my belt and so it's just MHO. Some sanitizer is prob. better, but one time without is fine.
A while back I brewed a porter in a bucket. My bucket is 7 gallons, but I didn't get the boil off I was hoping. So I put 6 gallons of liquid goodness in he bucket, pitched and sealed, and woke up to a bulging bucket. The CO2 could not leave the bucket quick enough. I tried a blowoff tube but my bucket system has a small airlock. So, I took off h lid and did "open bucket fermentation" for a day or so. Long enough to allow the pressure to subside but short enough so that there was still positive pressure when I put the lid back on.
It was a great brew. And I think it had nothing to do with that method