Is the house listed with an agent? Will you be negotiating or do you think the house is reasonably priced? If you like the house and price, it can be beneficial to use the same agent as the paperwork becomes streamlined. However, keep in mind that unless the agent is working as a dual agent, and discloses that fact, then the agent is working for the seller. If you will be negotiating, then I suggest you have an agent represent your interest. Especially since the seller almost always pays the buyer's agent. So why not have your own representation?
If the seller does not have an agent, then you can probably save time, headache, and money by asking an agent to represent you. In many states, the agent has standard purchasing contracts and relationships with attorneys, title companies, etc. which can get you discounted fees and have one "central" person taking care of all the tasks involved. Also, even with for sale by owners, the sellers are almost always willing to pay the buyer's agent fee.