14.5 inch wide loadcenter cabinet and hit the spot adjacent to where AFCI's or GFCI's are installed and you may have a problem. Otherwise there is no one size fits all rule that prohibits entering the side walls of the cabinet - it will depend on what clearances you do have.
Meter cabinet - if it is an underground feed style cabinet - there is usually plenty of width to easily install conductors through side wall. Some narrow meter cabinets designed for overhead feed only could give you some troubles. Those narrow cabinets are nearly non existent in this area as new equipment - not because they aren't made, they just aren't approved by POCO's and/or there just isn't demand for them anyway.