i would say ockam's razor is clearest and most called for when the context makes its use in-line with the corollary that the more extants, necessary states, whatever, you propose to explain something, the more likely one of them will be wrong. simple statistics. in a scientific theory, which has no purpose but to be a model that sponsors accurate prediction of empirical observation (while presuming that this makes the model true), ockham's razor is harmless. although i guess it could be vague which explanation is really simpler. to give an example outside of the domain of science, let's talk about the people who deny the existence of ghosts and spirits on account of ockam's razor behind (or in front of) their backs. this domain, incidentally, is where ockham's razor is no longer harmless. it's used to justify closed-minded thinking and the desperate holding on to the belief that one basically understands reality. i like to say that it's a double-edged sword. you can assume that it's more 'parsimonious' to leave out the postulate of ghosts, or spirits, or a spirit world, because you can explain the accounts of ghost sightings as ... delusional, lies, embellishment of memory, hallucination, *mass hallucination*, or dismiss them out of hand as 'hearsay', and explain reality with particles and the four forces. OR, you can assume that it's more parsimonious just to assume such a realm exists, and keep all the accounts of sightings (or, more generally, mystical experience, obe's, etc) simple, i.e. they saw or experienced what they say they did. rather than assume each and every one is a twisted farce. there's no way to formalize the likelihood or implications of etiher of these choices in order to compare their likelihoods. i think more specifically, ockham's razor becomes non-harmless when common sense and sennsibility is in any way involved in the options. or maybe it's just about real intention. 

i am not sure if there there is a black-and-white way to distinguish when ockham's razor can be used clearly, or if there is ever (except in rare or possibly only hypothetical cases) a black-and-white way to distinguish which explanation is more parsimonious. but either way, sometimes it's really clear. copernicus' explanation of the earth revolving around the sun and the corresponding math used to predict the celestial bodies was a hell of a lot simpler than what was before it. einstein said that if you can't tell the difference between acceleration and gravity then they should be regarded as equal in every possible way, which made an elegant general theory of relativity. although, admittedly, it's not necessarily true that 'space' is warped in the fourth dimension, supposing this even has any meaning beyond the mathematical. 

well, i guess that's all i have to say about that. ockham's razor is useful but razors should be handled carefully.