When you lay your fabric out on your cutting mat make sure that the fabric lays evenly on one of the lines at the bottom of the mat and also along one side. This will mean that the fabric is squared on the mat. Then when you use your cutting ruler be sure your ruler marks are even at the top and bottom of the fabric according to the lines on the mat. When you cut this way everything is in square and you won't get angled cuts. The rubber cement tip is a great one and I've used that for a long time. My ruler doesn't slip at all as long as I apply even pressure to the ruler. I use a 45 rotary cutter blade, I found the larger the blade the harder it is to keep it rolling straight. I also have some size 20 cutters and blades that I use to square up blocks. That might just be me but I find the smaller sizes to be more accurate when I make the cuts.