When soldering MCU (Arduino and like) to perforated board I apply solder only to the pins in use, leaving the unused ones just sitting in the hole. I do this to minimize soldering accidents. The headers themselves had all their pins soldered to the chip.
It's this a good practice?
No, it's not good practice. Obviously, the header will be better fixated mechanically if you solder all pins.
For this kind of task, I'd expect very little soldering mistakes to happen, so go ahead and do it.
Furthermore, this is confusion waiting to happen if you decide later on that you want to use a pin you haven't soldered yet.
That's fine for prototyping, the only downside is the reduced mechanical strength.
You can do it, but you have to take mechanical resistance into account.
What I do is solder all the pins I use, plus pins on both ends. on a 2.54mm pitch (like the arduino headers) soldering accident should not happen (assuming decent equipment and careful operator), but not soldering every single pin makes rework/desoldering easier.
Since an Arduino is prototyping work, mechanical resistance does not have to be perfect anyway, and you can always solder the rest once the project is finished.
