Taking the whistle only first time sounds like a good idea, for purposes of making an inconspicuous entrance.

On the other hand, arriving as a bass player has its own merits:

1. It's *your* first instrument, after all, on which you're most comfortable.

2. As a bassist, you're an accompanist and should be able to play with anyone/anything (as long as they don't capo to C#!)

Only hitch would be if there's already a bass player on the session; two would be awkward. Also, you're not well positioned to *lead* a song/tune of your own, and shouldn't be expected to. Your role would be different from that of all the singer/guitarists, and even most other instrumentalists.