I think a good rule of thumb is to always use your agent as they have your best interests and represent you. If you have a good agent and a good relationship with them, don't risk creating a misunderstanding by leaving them out of a step. As has been previously mentioned, there are a set of rules and ethics for agents and brokers that should always be maintained if you want to be respected by your team, and as a professional in the investing world.
I would have this discussion with your agent. Let your agent express their feelings and concerns. If you think your offer might be a lowball, and you don't trust your agent's advice on whether it's worth the try, then evaluate your objectiveness or get a new agent.
There are so many potential problems that can arise if you talk to listing agent or sellers agent prior to speaking to your agent. Your offer will look more legitimate and be more seriously considered if you have a buyers representative interacting for you anyway.
With our agent we trust and respect her input on appropriate offer ranges. She has helped us many times with factors we may have not been aware of or have forgotten. She has additional info from the MLS like agent notes, and In regard to price, location, jurisdictional issues, market conditions etc.... The only time we consider making a direct offer is with a home seller who responded directly to our marketing and doesn't have a listing agent. All the best.