As long as you are wearing sandals/shoes that can get wet/you don't mind getting wet - you can wear those when you think the chances of rain are high. Better yet, I have done it a few time, wear your sneakers out (I do now most of the time especially for the parks) and throw a pair of flip flops (thin/light) just in case you get caught in rain that might lasts more than the time you are willing to wait it out in a shop, etc. Wearing flip flops all day in the parks is not good anyway. I learned my lesson years ago when I did it for one of my trips.
I always have my umbrella and ponchos but sometimes we are just caught in it. We forget or don't think we need it.
Good luck and have a great time!!!