We pretty much determined if events were important based on the level of personal accomplishment, effort and achievement. For example: kindergarten graduation. Most kids will graduate and get their little moment in the spotlight simply because they were a participating part of the class, did the work that was expected, and didn't get expelled - let's be honest. But for another kindergarten child who's fought cancer, or who suffered a tremendous tragic loss, but who managed to still participate in kindergarten and graduate, well, that's something the whole town should attend! (At least the whole family).
The same thing applies to concerts, and similar events. If the whole class is singing a little spring concert, regardless of ability, then it's ok for just one parent to go, if possible. However, if it's a music group that your student had to audition for, rigorously practice for, and has a solo in the upcoming spring concert, then more of a family effort should be made.
When my ds graduated from high school, only me and dh and dd attended. School was easy for ds, and we didn't even throw him a party. We gave him a laptop for college, and after graduation was over, he was off to a round of parties all over the town. We knew that and anticipated that and were fine with that. That was the kind of kid he was (and is): lots of friends, pretty easy time of getting a great GPA and getting accepted into college.
However, when dd graduated from high school, after 7 long, long years, with about a 1.0 GPA, we threw a party like the presidential inaugural ball. Her graduating was a massive achievement - with all her medical problems, hospitalizations, the days she attended her online classes when she couldn't even lift her head off the pillow, the effort was amazing. We invited tons of people (who all came), and we took everyone out to dinner and made a huge deal of her - dh bought her flowers, and people brought stuffed animals and flowers and gift cards.
So, bottom line. Is the event an "everyone pretty much has achieved the same thing" or "is my child graduating or performing because he's in kindergarten or first grade or eighth grade and everyone in kindergarten or first grade or eighth grade is also graduating or performing" or is this an event where the child or student has persevered against the odds, and has overcome something (whether its anxiety or illness or injury or trauma or learning disability, minor or major)? That was how we decided what to attend and what one of us could attend, and usually it was me because dh was working. He did attend one of ds' middle school events, but it was something ds won an award for and would be personally recognized. Other than that, he didn't take time off. But we always made sure to observe the occasion that evening - with the grad's favorite dessert, or going out for dinner, or an appropriate gift, etc.