i try to match the thread with the (main- feel) of the quilt- quite often i use a medium gray or cream--
if the quilt (reads) white- and has a white back i use white thread to quilt it-
i find i get the best results when i can use the same thread top and bottom- but sometimes that is not possible.
i either match colors= or pick a neutral that will disappear- or use a varigated that changes all over the top- which can add some great movement...
i usually choose 4 or 5 different possibilities- pull some out and lay them all across a section of the quilt and then i walk away- go back a while later and look= decide which seems to accomplish what i am hoping to accomplish.