I normally do just a single horizontal and a single vertical to mark the centre as I don't have the patience to do a proper grid. Way back when I used to do a grid every 50 rather than 10, but I find that I make more mistakes doing the grid than doing the design itself.

I just select a different colour thread - not necessarily machine thread but that is nice and easy to remove - and then stitch over it. Once I've completed the design I cut the guide threads on the back and just pull them out.