Note that the passive convection cooling requires a large temperature differential to work. This means, you need to design the electronics to run (including proper lifetime analysis) at higher temperatures than with forced (fan) cooling.
In practice, it's hard to design the heating components to run much below 100 degC with passive cooling, while with fan cooling, it's fairly easy to design them to run at, say, 50 degC (assuming you specify maximum ambient temperature at around 30 degC). This is, of course, assuming that significant cooling efforts are needed at all.
The step #1 is to calculate the actual heating and approximate the cooling through the surface area of your case to see if you need the holes or not. If it runs cool anyway, why bother?
Often testing and measuring is much easier than doing even a crude calculation.
For example, if your components happen to run at, say, 60 degC with closed case, there is no point in adding holes because it runs cool enough anyway, and with such low temperature differential, air wouldn't circulate through the added holes anyway, so they would only bring dust in. But if you measure your components running at 120 degC, nearly destruction, adding a few tactical holes could easily bring it down.
Run the device long enough so that all air inside the case has time to heat up. This may mean hours, or over 10 hours in a large device.