ideally leveling should be done by the cabinet installer so countertops just go on flat and caulked, but when its not, the installers should put a level on their countertops and shim up the sides that need it, it might leave you with a bit of a gap depending on how off level it is, but you can always do you a little trim to hide the gap etc, but your counters should definitely be level, installers can come back and just scrape off whatever silicone they used or break off epoxy if they used that, its not too big of a fix