i generally decide what i want for the top- be it compimentary, contrasting, verigated...what ever i choose- then i choose a thread for the back- that again either disappears/blends in, contrasts, compliments...goes with the back- about the only time my thread is the same top & bottom is when my background is the same- like a black background quilt with a black backing will have black thread- or same as white top/backing...i pretty much never quilt with invisible thread- i save that for applique- or some technique that requires it....i've had to re-quilt too many quilts for people who used invisible thread to only have it break apart- sometime down the road. i know some people have good luck with it- but i've not seen it hold up on a quilt that is to be used as a quilt- only on wall hangings. (just my experience) i would recommend matching your backing thread to your backing- and use what you like for your top- i think when people say you should use the same top & bottom it is more the (weight/type) of thread- not necessarily the same color.