Assuming the kilt is for an average sized man (hips/waist under 46") I never cut out the fabric in a four yard kilt. I rarely cut out the fabric in a five yard kilt. I almost always cut out the fabric in a six yard kilt.

So my rule of thumb is that I expect to cut out the inside of the pleats on anything six yards or greater. But the final decision is really my judgment call after sewing up the pleats. I can feel the bulk of the cloth and just tell if it's going to be too bulky for wear without cutting out the pleats.