I have done hundreds of a/c tune-ups in cool to cold weather. I remember a few where I washed outdoor coils standing in snow. The only thing about blankets, cardboard garbage can lids is that you are simulating hot weather, but what simulation temperature are you simulating? 80-90-110??? And then there is no load inside. Don't add refrigerant! do your thing, clean drains, coils, filters....... and just hope charge is good. Company's do it to keep tech's busy and try to avoid the big rush the first days its hot out.
As iron sharpens iron so one man sharpens another Proverbs 27:17 NIV84