Personally, I use isopropyl... it's already mildly poisonous so they don't intentionally add any methanol or other junk to it. It should be just isopropyl and water.
Also, in order to go blind from Isopropyl you'd need at least a few grams of it (look it up)... Which means unless you intend to smoke at least 50 grams of isohash in one sitting where it's still heavily laden with alcohol, you won't have to worry about it. I figure my iso-hash is maybe 10% alcohol at worst(probably more like 2%) and I only need .1g to get stoned... so .01g of isopropyl.
I don't know about you, but I'd rather smoke 1/1000th of a dangerous amount of isopropyl than take a chance with a lot more dangerous shit in butane. A few hits of iso-hash probably has less isopropyl fumes in it than taking a sniff of the bottle.
Besides, once i have my isohash, I could always add a small amount of 151. Almost every bit of the isopropyl would be replaced with ethanol by the time it dries (maybe an hour later?) and I have no problems with inhaling ethanol... IDK if you can remove w/e impurities remain from butane just by adding ethanol.