sharp knife is what I use to prevent curling of the edge as you can sometimes get with scissors. score it a few times not trying to cut through and usually it will break off clean when bent on the score line
I have used both methods and find it easier and faster to use scissors. The down side to that is that the strip you are cutting off will tend to curl up. but at 0.3 mm she can easily be straightened out.
One way which I am doing it recently is to cut the brass plate on the circular saw. The width of the strip will be consistent and I don't have to worry about the curl-up. For thicknesses like 0.3 mm I apply masking tape on both sides. Obviously metal-cutting saw blade has to be used (which is also useable for cross-cutting of timber)
Another more advanced technique is to use a table saw with an adjustable blade hights. You would use one of the Jewelry slots saw blades 0.2mm thick. Tape one side of the brass plate with masking tape before proceed to cut. The results are awesome, however, it clogs blade with tape glue residue and dulls the blade. I use this method when I made gunport hinges: Fists I solder the tube on the brass sheet and cut altogether.
I think this is a problem, a lot of modelers will have different methods and results, every time based on the material and also the tools they have at home or in workshop.
I would take a piece and would try with different tools you have by yourself, and I think you will find your best method very fast