Years ago when United had an exclusive relationship with Hertz/Dollar, there were often good United miles bonuses available, at least for UA elites. That relationship ended; not really sure if any other airline picked up the slack and is offering similar Hertz promos.

In general I find standard miles earnings not very appealing since the rental agencies charge a surcharge fee that cuts into the value of the miles earned. Occasionally it may make sense to credit a rental to an airline or hotel where I need activity to keep the account alive.