I quilt on a longarm and I use whatever gives me the look I want. Sometimes I want the back to have the high contrast so in those cases I will match bobbin thread to top thread. Other times I want all the quilting in the back to blend in or I don't want to be bothered with changing the bobbin thread every time I change thread color in the top, then I will use a bobbin color that blends in or compliments the back and whatever color looks best on the top. It can get challenging when the top and bottom are really high contrast, like a white in the bobbin and navy blue in the top and then add a different thread type or weight. It takes a good amount of tension tweaking to avoid the pokies and there are so many things that can cause them to suddenly appear like a blob of lint getting caught in your bobbin tension spring, things will look great on top but when you look at the back there are pokies. That is why so many prefer to match bobbin thread to top thread in color. Also, with LA quilting what can often happen is if you have sporadic pokies, once the quilt is off the rack and washed, the top and bottom "tug 'o war" equalizes and those pokies sink into the batting like they are supposed to.