You don't have to strip the insulation off the copper wire before selling it to a scrap metal yard but if you do remove the insulation from the copper wire then you'd get $1.00 more per lb for the scrap copper wire.
Some scrap copper wire is too hard to strip and not worth it so I sell some as is with the insulation but any wire that is easy to strip the insulation off I will strip it and sell it as clean copper wire for scrap.
If you sell the scrap copper wire with the insulation on it you'll get around $0.40 to $0.50 per lb and it will be bought as insulated copper wire.