Should I have a medical before I make an application for Life insurance?
No. An application for life insurance is based upon your current knowledge or best knowledge to date, of your medical situation. If you honestly don't know of an emerging health situation, you cannot be expected to be able to disclose it on an application, so an insurance company can't simply exclude the unknowable from your cover.
If you're required to have a medical, as part of a life insurance application, or due to the level of cover you're applying for eg: $2,000,000+, that will be arranged for you anyway.