Normally you should employ a wireless site survey that takes in the business use case for the wireless network and the used device types, user counts etc... to determine the number of AP's and the best suited locations.
Usually in an office environment AP's with internal antennas like the MR36 should be mounted horizontally so you get signal spread around the AP. Other environments like warehousing, production, outdoor can have way different placement. This is why a site survey is important.