When you start a long cut, put pressure on the lower half of the ruler. Then stop cutting, walk your fingers to the next section and keep cutting. If you try to control the ruler for a single cut, it is easy to shift the ruler.
I make sure that my fabric hangs properly when held salvage to salvage before I fold it to cut. That avoids the "Vs".
When I have to make multiple cuts, I add the widths and make one cut to that width before sub-cutting that piece.
Example, instead of cutting 4 strips of 2.5" in a row, I cut one strip of 10" and then sub-cut that into 5" and 2.5"
increments.
If I keep making separate cuts, I check periodically that my fabric is still square. You'll be amazed how much shift you can accumulate.