My company generally allows using the APU as required.
However, this does not mean "fire at will", as this does not only involve airline or onboard procedures. Individual airports have their own preferences and procedures as well. At some fields that tend to be closer to the equator and smaller in size, it is possible to run the APU and on-board AC as You see fit; other seem to abhor anything audible and running the APU outside their stipulated timeframe after on-block and before off-block may even carry a fine. Those fields, however, normally offer a ground-based air conditioning unit of rather variable quality.
If on such a field, the ground-based AC is inop or unsatisfactory, there usually are ways to quickly obtain a tactical dispense from the rule and be allowed to use the on-board systems. Be it a simple request via radio, a phone call to a number available in the airport documentation or a mere request to the ramp agent - all those have worked quite well in the past.
Then come the abnormals. The question whether or not one is willing to accept an aircraft with an U/S APU for a flight to a destination where its cooling capability is needed, is to be well considered.
Another thing, as You spoke about summer: the ramp is a hot place to work. A bottle or two of cold water from the galleys is sometimes easily spared and, if taken to the ground staff, often greatly appreciated.