GSD selection

Is there a desired GSD for say a thermal project, or should it merely be chosen based on how accurate the final model needs to be?

From my experience the GSD is chosen based on how accurate the outputs need to be. 

There are a few exceptions where you would get better reconstructions by adapting the GSD. For example for dense vegetation areas (e.g. forests,…) a GSD of 10cm/pixel generally gives better results, because it simplifies the complex geometry of the vegetation. 

Here is a tool that can help to compute the GSD: