I use pine for all mine. for the box joint I use a router and a template. cuts super clean and is plenty fast, I stack numerous blanks next to each other, and go through all of them at once, to the depth of the tail/board which is usually 3/4"
my normal sequence of steps is using my table saw to rip the edge of the board to square it, then rip the other side to size and square it, then cut all the same sized blanks for say the length and then do the same for the width. I then router the rabbit cuts where needed (it is important to do this now.) stack up to 10 usually opposing sides (because they will have the same joint pattern and are the same size) and clamp them together and use the first template and run my router alongside all of them, this makes the first inside cut, then I put on the second template and do the other side. I make 1" joints spaced one inch apart centered to the board. the outer tails are larger and take up the difference. This is why you want the tails of the one side to be at the tops, because you can router straight across for the rabbit cut, then the tail finishes it off square. ok, you do not 'have' to have it square on the rabbit cut it can be rounded if your frames are dogeared, which they should be, because it makes them easier to work with, and there is no reason not to make them this way.
I also made a simple jig that is square and the correct dimensions for the inside of the box. it is two pieces of plywood spaced 6" apart by a block of wood and on a base. All I do when I am assembling is use a glue brush, smear glue in all the joints, put them on the jig, secure with quick clamps, and then use my brad nailer to shoot 1.5" brads into each tail along the side, pop , pop , pop , pop remove it, wipe the joints clean and it's done when the glue dries. it costs around $7 - $10 maybe.
If you do not use a jig, then a simple square will make sure its square.