Yes, testing with something is a good idea. Don’t have to print it, probably enough to check what it looks like in the slicer. If you output an .stl in ascii, you can also open it in a text editor and see if the values makes sense.
The unit doesn’t export, only the vertex coordinates. Default cube is 2x2x2 BU, which in metric is 2x2x2 meters. Exporting it will put coordinates like -1, -1, 1 and 1, 1, 1. When you import it, you could tell the application it’s in millimeters, and you get a 2mm^3 print. If the coordinates are an order of magnitude off, then something went wrong.
You can pretty much forget about the unit scale. The scale can be applied with that too when exporting, but you don’t need it for working and it’s easier to just look at the object dimensions and make sure its scale is 1,1,1. Which you have to do either way.
The viewport has clipping settings, which will need adjusting if you start working too small or too big. If you model for 3D printing a lot, and the scale you work in doesn’t change much, you might want to use an actual unit, perhaps even set up the startup scene so it uses mm by default.
Using an actual unit does give you the ability to input with an unit, it also does conversion so you can input 3in even if you use mm. Blender unit, or none unit, means you can decide and start working right away - It could be mm, cm, inches, or if the reference photo has a coin as size reference, could also use currency as the unit of measurement. Switching to real world unit then scaling or setting the dimensions afterwards should be straight-forward enough.
Most input fields can also do calculations, which can help when sizing the object. It also knows what pi is, if you need to input a circumference.