Only if the seller asks! Once I saw a noted hybridizer "go off" on someone who innocently requested a specific bonus plant to him while we were visiting their garden. Yikes! That made an impression. The hybridizer's point was, "we don't HAVE to include gift plants and are doing it out of kindness, so it is greedy of you to ask for a certain plant." My order hadn't been dug at that point and I wasn't about to mention a bonus after that!
Often I send extra fans of the plants ordered, so buyers should always keep in mind that a gift is just that, a gift that the buyer can keep or if they don't want it, maybe give it to someone who does.