I generally miter all my borders, ((serious OCD here)), and always cut mine bigger to make the corner. I measure the length of one side, cut/piece the border for that plus extra for the mitering. Then pin from center out. I havent had any trouble with wavy borders, except when I dont miter properly...sigh that makes a mess!
I have cut my border strips too short before by 1 inch or so,
those quilts usually end up being scalloped border