I'm transporting a wedding cake several hundred miles.  I have the tiers covered and ready to go.  Would it be better to add the supports now, or after I arrive at the site?  I use bubble tea straws. 

Shouldn't matter. I'd say before though just cuz the more you have done beforehand the better.

Thanks for your reply!  So, you don't think potential jostling in the car will move the straws around too much?

AThe straws will help add stability to your cake. Put them in before the drive.

Yeah Laura, I agree with the other posters - have this done in the comfort of your own kitchen before you set off. Also, make sure you have a non-slip/grip mat (from any hardware store) to set you cake boxes on before travelling so they don't fly all over your trunk.  Best of luck!

Assemble before...one less thing to do.  And everything else the above posters mentioned. 

Best of luck with the travels.