Talking with mechanical engineers, I find a range of criteria being used to determine the cooling load of electrical equipment spaces. Estimates of cooling loads based on equipment efficiency range from 1.5% to 5% of the kW being served by the space. Others use rules of thumb for cfm/sq ft, or W/sq ft. All would prefer to receive calculated values from the electrical designer, but they usually don't.
In many cases, electrical equipment load is insignificant with respect to other space loads, and proven rules of thumb are reasonable and cost effective. However, in large, concentrated installations such as substation rooms and data center back-of-house, loads are significant and the cost of errors can be high, so I would make a case for more accurate load calculations.