You're raising a very interesting question. It's somewhat difficult to determine. I know that some provinces are much further advanced on certain issues that would be relevant to sentencing; for example, fetal alcohol spectrum disorder. Some provinces really understand what it does to cognition and capabilities.
I would say there is not a monolithic way in which people are dealing with mental health issues across the country and the way in which they are being recorded or supported in pre-sentence reports. This would help to standardize the practice.
The “wherever possible” gives a considerable amount of room, but what it does is flag that this is an issue you really should be looking at. The probation officer, in the interview with the individual, should be asking them whether there's anything they would like to share about their mental condition or about what they were thinking at the time the offence was committed—things like that—so that they can get some sense at to whether there are, from the offender's perspective, any relevant issues that should be pursued in the pre-sentence report.