On anteriors I dont get it when I sprue on the incisal edge but I do alot of posterior press and stain and there isnt an incisal edge and it seems like no matter where I sprue I get that blister or spot of more trans, so I sprue all the posteriors from the lingual.
On lower anteriors where I press and stain and dont want to mess up the incisal edges or when I cut back for mamelons I sprue on the lingual just below the incisla edge.
Its not the correct place to put the sprue per the manual but it works for me.