I've got a magnetic bag but have always used straps on it when using on the Bonnie. Reason being that the elevated gas cap on the bonnie allows air to get under the magnetic flaps and the bag will fly off at speed. Ask me how I know.
BUT... I saw that Newbonneville.com is now offering a lie-flat gas cap which I've requested for Fathers' Day. When it comes, I plan to go for a spin without the straps on the tank bag.
Will let you know how that works out. Looking forward to losing the ugly orange bungee cord I've been using the past few years.