This is the first thing I teach my "newbies" along with the importance of using good quality threads. We have a deal with a local shop to buy by the box and get a discount. That gets us good quality at a reasonable price. I recommend ivory, medium grey and black/dark navy.
If you buy your threads in pairs (two reels at a time) wind one whole reel onto bobbins and put the other through your needle. That way you will finish both at about the same time and not be left with all those "bits" on reels/bobbins that are just a nuisance to thread through the machine. What little is left on the reel or bobbin will do for small hand sewing jobs. And you will have your replacement bobbins ready to go when you run out mid-seam without having to wind bobbins
If you sew on and off a small scrap instead of pulling out and cutting threads (top and bobbin) you will save up to 2-300yards on a large quilt - that's another reel of thread.
Until recently we in Oz have not had ready access to the larger cones of
50wt threads such as Aurifil, but that is changing. I have contacted the Australian distributor for Aurifil and we are now ordering neutrals for our group, and thereby saving on postage as well.
The exceptions to using neutrals for piecing would be if the entire quilt was in shades of one colour (in which case the quilting and piecing would warrant buying a larger quantity - box of reels or a cone - of a specific colour), and if the quilt is predominantly red. I do sometimes choose red thread for piecing red fabric. But if your thread tension and stitch length are correct you shouldn't see the piecing thread.
Our group also runs a "thread stash" of colours. That way if one of us needs just a small amount of a special colour we take it from the "stash", return what we don't use and throw a few dollars in the kitty to buy replacements. This is a big saver when quilting designs such as Round the World, where there can be a large number of colours to be quilted, but only a small thread requirement for each colour.
Every little bit helps. And by saving on piecing threads (without sacrificing quality) we can "splurge" on those more visible quilting threads!