Once upon a time, I was a newbie to querying agents. I fretted over every word in my query, dragged my heels about sending it out, finally did at the insistence of my boyfriend, received some requests, and every little thing sent me into a panic. If I do this, will the agent hate me? How should I respond to this? Or this? What do I say in a cover letter? Is it okay if I send the manuscript media mail? Do I put a rubber band on it? If they want internet files, do I put them in a folder? Or just send files loose? Should I tell them I'm working on something else? Should I send this revision?
Every new thing to come along provoked a freak out. For awhile. However, my query search last for three years. Many many queries. Eventually, I grew casual. Professional, but casual. I just went by the rule of thumb, "Would it bother ME if someone did this?" And, "If it does bother this person, would I still want to work with them if they were bothered by such things?"
As a querying author, or an author who intends to query someday, or even an author with an agent on submission to editors, you should keep up with what's going on in the industry, sure. You should be aware of standards. And you should follow guidelines. But please remember, agents ARE just people. Some of them will be more forgiving than others, some will be annoyed by X and not by Z, etc, and there are a lot of good agents out there. As long as you conduct yourself with as much common sense as you can muster at the time, there will be an agent for you even if you make mistakes.
Some of the things I did while querying that could be considered mistakes:
--I replied to my first "positive rejection" with a gushing email thanking the agent SO MUCH and telling them I'd be querying another project soon (aaaand about a month later I did...she didn't ask for pages)
--I emailed several agents who had had my full for a month letting them know I had a MUCH BETTER version now
--I rushed a revision and queried too soon
--I emailed some agents three times with three different versions of the Magic Under Glass query over the course of two years
--I queried some agents that I'd heard kinda bad mojo about because I knew they HAD SOLD SOMETHING
--I talked about most of my submissions and rejections on my blog, plus posted teasers regularly and blabbed about the story and plot all the time
At the time, I used my best judgment with the information I had available at the time on how to handle a situation. Sometimes, in hindsight, it turned out to be wrong and I stopped doing it. But I never did anything that was so heinous a crime that I was blacklisted from all agents ever. In fact, no one ever really seemed overly annoyed or whatever and I always had plenty of requests (in fact, one of the agents I kept pestering with new revisions ended up offering representation, although she was not the agent I signed with), and in the end I got a lovely agent.
Now, I know there has been some various debate on whether posting excerpts of your work or reviewing books (especially critically) on your blog, etcetera, can hurt your chances to sign or sell. I think it is definitely wise to discuss these matters, follow these discussions, and think about what you might be saying and who might be reading when you blog (or Tweet, etc) and if you feel comfortable with what you're putting out there. But please, do not go overboard. Again, use common sense. "Don't talk about the submission process" doesn't mean you can NEVER tweet "aw, man, I got a rejection", it means, you might not want to post a breakdown of everyone who rejected you.
And even if you DID make some manner of error in blogging about all your rejections or really snarking about books or posting a chapter of your WIP, and now you are freaking about it... Well, trust me. These things fade and they never bothered some people to begin with. If you feel that what you did was a bad idea in hindsight, delete the posts and continue on your merry way. If you feel that you are doing what you want to do, such as honestly reviewing books, continue on! Just as you can't please all of the people all of the time, it is not that easy to piss off all of the people all of the time. Unless you are running around making a general jerk of yourself on a regular basis or are remaining willfully uninformed, you're probably fine, especially because social networking IS relatively new and people are still figuring out how to manage it and use it and what's appropriate and all that stuff.