when the quilt top is done is when to choose the best batt for your quilt-
fusables, sprays and pinning are personal choices- but from what i've heard the fusables are better suited to smaller projects (just what i've heard-i've never tried fusable batts on anything larger than a table runner)
also- different batts require different quilting- some have to be quilted every 2"-4" some allow up to 10" between quilting lines- some are well suited for tied quilts- others are not- it is important to read the package and choose the one that is suited to your quilting plan. and size- after the top is finished you need to measure it-then choose a batt that is a bit larger than your top.