It depends on your strategy, and availability of appointments for medical examination. If you are sure you want to enter Canada as soon as you get PR then get your medicals done sooner.Hey....does it makes sense to go for medical exams near the end of 90 days period for example after 65-70 days after receiving ITA so that one has a bigger time frame to plan their landing in Canada.
If your strategy is to have a PR as a backup option and explore other avenues (such as working in another country, or getting maximum revenue out of your current job etc.) then go for medicals 65-70 days after your ITA, that way you'll get maximum time to take care of your pending matters before you enter /land in Canada