Better a cummerbund than a gash of white between waistcoat and trouserband, and better either than showing where the facings of the dress shirt end---and better that than showing an untucked shirt. If you are tall enough not to be able to find waistcoats then a cummerbund is very useful, even as you get fatter.
Traditionally evening wear demands silk socks, but they're not robust enough to tuck your braided trouserlegs into while riding, so I stick with long fine woollens.