Apply at least one month prior to May for your extension, which would give you enough time and you wouldn't have to stay inside Canada for 90 day grace period.
OK, so your study permit will be extended to the date of graduation of the program that you have given. Once you have graduated from your college you have 90 days from the day of your graduation or from when your final marks was given, whichever comes first to apply for PGWP. You will not gain any additional days for applying during this 90 day grace period. Your PGWP will be dependant on the duration of the course you have completed and you will get the PGWP depending on the date you graduated and not the date you applied for the PGWP.
To reiterate, there is no benefit from delaying your PGWP, do note that your study permit has to be valid when you apply for PGWP. You can work until your PGWP application decision is made. Do it ASAP after graduation and you can start working full time after applying.
Your study permit extension has nothing to do with applying for PGWP, it would be better to extend your study permit now since you need a valid study permit to apply for PGWP. Get the extension and once you graduate, apply for PGWP.