Shape of logs of insect screen frame
The 45 deg cut gives more surface area for glue if you glue and more strength , 90,s are easy and can be nailed or glued but any paralyx (not totally square) will show more and be weaker than a 45. I have done both in the past but 45's look professional and last longer. Just look at your door frames, sliding screen doors and you will find 45's not 90's