For me it depends on the design, really. I like to start with the colors closest to the fabric color so if I have to make a float with contrasting thread it doesnt show through that easily.
Its an interesting thread! I prefer to start in the middle to make sure Im not out of canvas somewhere:) But starting in the top left corner (to keep the finished stiches as clean as possible) is a great option too.
It depends on the design for me also. I also use a frame where the finished side is rolled to the inside of the frame so it is not exposed to getting soiled. I use dowl rods to get the different widths for my patterns. I initially bought a frame in order to get the side pieces but now I can replace the rods with the dowl rods and make a frame up to 36 inches wide in order to sew without getting creases or raised areas in my patterns from having to change the rod positioning. I simply roll it head to toe and never have to move it from side to side