It is of course perfectly legitimate to follow a different order for roleplay reasons or because you want to add a specific companion or items to your party sooner.
The preferred order to visit each area is as follows, based solely on the lower and upper limits of BioWare's hard-coded challenge scaling for each area. However, lower-level players may find these locations rewarding in both experience and higher-tier items. Both encounters will be doable, if not downright easy, if you come to these areas well after level 10 as intended by BioWare. Likewise for the Ser Landry encounter (the duel in the back alley) in Denerim.
BioWare meant for the Bounty Hunters outside Orzammar as you approach it for the first time to be a "gating encounter" to scare you away if you're still too low level for the intended challenge level of Orzammar. Going to these higher-level areas too soon is a contributing factor to some players reporting that the game is "too difficult" for them even on Normal mode. Unfortunately BioWare does not make this clear at all. Although the game itself allows you to visit Denerim and Orzammar immediately after leaving Lothering, you're not meant to do so until much later in the game.