@Wei-Der Huang
To piggy-back off of what @Russell Brazil said ~ it depends on what your goal is.
Are you rehabbing? (In that case I'd be talking to as many agents as possible who may have access to on/off market properties - especially if inventory is low). I tell rehab buyers I will "put them on my list" and as long as I know exactly what they are looking for, I can call them when a deal comes across my desk. Typically the agreement is if I bring them the deal, I'm also the listing agent when they sell the completed project.
Income/commercial buyers are a bit different. I'm helping them build wealth and it's more consultative. Generally speaking, I want an exclusive relationship.
Residential home buying is also different, IMO. If someone is looking in a specific neighborhood to LIVE IN, I recommend an agent who specializes in those homes/area. (But this is just me ~ I work primarily with investors to begin with). I want the buyer to get the highest level of service possible and if I don't know the market intimately, I find someone for them who does.