I'm making my first stacked cake in a few weeks and am planning on baking/decorating a few days before the party. Would it be ok if I stacked the cake the day before? I'm worried about it sinking. I'll obviously have dowels in it but wondering if that might be a bit too long to leave it...

If your cake is structurally sound and your dowels are all properly cut to the same height you can definitely stack the day before.  I always stack the previous day, stacking and final decorations on the day of the function is way to nerve wracking for me and gives you no time to fix anything or slightly change anything that is not quite working in the design.

I agree. Get as much done the day before so you can concentrate on last minute details the day of.

As 810whitechoc said, if you support system works you have nothing to worry.

 I always worry more about the freshness of the cake... Unless it's a huge wedding cake I would not start "a few days before".