So, I've queried 12 agents in the last 2 weeks. No responses yet, but that was expected, everyone says 6-8 weeks. On the big checklist, I've got 7 more that might be good, 7 that might be okay, 13 that are respectable but that I'd rather avoid if possible. Something occurred to me yesterday. Suppose I get a nibble, but I get it from one of the lesser agents, and I still have hopes of landing one of the bigger places. Can I just not send a response to the lesser agent until/unless I get a rejection from the greater ones? On the other hand, what if I say yes to the lesser agent, and then a week later one of the better ones says she's interested in my MS? I don't want to be rude, but I also don't want to make a professional commitment that would turn out being a suboptimal career move.
First of all, you shouldn't be querying any agent you wouldn't want to work with. Period. It's rude, a waste of their time and your time, and counter-productive. If you wouldn't want to be represented by the agent, do not query them. No agent is better than a bad or substandard agent.
Second, I'm not sure what you mean by "nibble." Do you mean requests for partial or fulls of your manuscript? Or do you mean offers of representation?
If you mean partials/fulls, as long asthey are not exclusive, send them. And keep querying.
If you mean offers, well, then I refer you back to the "First of all" above. But then I say, if you are lucky enough to get an offer of representation, you should: (1) respond promptly and professionally to the request to talk/offer; IF YOU ARE INTERESTED IN BEING REPRESENTED BY THE OFFERING AGENT, then, (2) you should tell that agent that you have partials and fulls out and you'd like to let the other agents know you have an offer and give them a chance to respond; and (3) then, let the other agents know. They can then make an offer of their own or bow out.
But, again, do not query any agent would do not want representing you.
AND, if you do sign with an agent, then you can not keep querying. So...there won't be chances for other offers. And if a stray agent responded to an old query, you'd have to tell them you are represented already or break with the agent you just signed with to pursue the interest.
There could be a number of problems with breaking with your newly accepted agent - starting with you'll likely have a contract and ending with damaging your reputation in the industry, and a host of others. BUT, you could certainly consider that.
Research agents carefully. And only query ones you are genuinely interested in. And if you are not sure, then query the others you are sure about first.
ETA: And, of course, once you have interest and offers, then you can talk with the agent and make sure they are a good fit, you agree with their perspective and objectives, etc. But you shouldn't query any agent you think of as one of the "less desirable" agents.
~suki