Not really. If there is a posted standby list at the gate podium, and you can see your name on it, then theres no need really to check in with the agent as they have that standby list in their computer and will process seats as they get to each person.

If there is no posted standby list, then its a good idea to check in with the gate agent to advise them you are there.

IF its an oversold flight and you can see the agents are being swamped with revenue pax then yeah - not a good idea to interrupt them. But if they arent frantically trying to do something, just a quick little 'Hi Im so and so, just letting you know Im standby and Im here' to give them a heads up, is almost always appreciated.