Well, you can do medicals in advance if you want to but remember that the medical results are valid only for one year. So if your application is rejected or if there are delays in processing, you may end up having to do the medicals again.
It is believed by some that advance medicals will make your procesing faster. If you see in this forum, many applicants who went through the route of doing medicals only when requested have got PPR in 6-8 months. Some applicants with advance medicals have had delays. So doing advance medicals alone does not appear to make an application move faster (just my observation).