You have to be careful on one of the sides as there are brake or fuel lines (can't remember which)running close to those areas.
Almost all of the bungs running allong the sill on the nearside had traces of rust on mine, and it does spread under the underseal and you can't tell by looking at it.
To be sure, i removed all bungs and had a look. They weren't bad, but i cut any rot out anyway and welded a plate, to save grief later.
Most people wouldn't bother, but I was underneath in a pit, in a warm(ish) garage, so i carried on with it.

Cavity wax and epoxy resin paint should keep rust at bay.