i use the muslin back- have been doing it that way for 45 years- that's how we were taught way back when in 4-H
the layer of muslin does not wrinkle up- you place your embroidery square and a matching muslin square together and hoop them--and stitch-
that second layer keeps loose threads (or carry overs) from showing through on the front and adds stability and body to your piece.
using a fusable ... i've tried once---it was VERY DIFFICULT to stitch through it- caused sore fingers and (i only did one block that way thank goodness- cause i felt it ruined one block)
my granddaughter's all love to embroider and i've taught them to back their work with muslin too