I remember when I was applying to colleges and the advisor told me that a certain amount of my application was left to chance. If the admissions officer was reviewing it at 2 am after a long day they might just end up looking at it differently then if it was sometime when they were fresh. I liked to think better of the people in charge of my college-related destiny, and of course, I never found out if that was the case.
Yes, there are agents who admit that they might have rejected you because they were in a bad mood that day, or just too busy to deal with anything but some crisis with the author having a breakdown over attendence at his reading. However, you have no real reason to worry about this. If you have a great query and you send it to 20 agents (and 20 agents would be an absolute minimal - I would shoot for 50 at least), then chances are it's going to get more than a couple hits. Not everyone is having a bad day.
As for me, I'm the mail person, so I don't have bad days. Sure, I have days where I go to work sick, or anxious about an upcoming endoscopy, or nervously waiting for a call from a doctor, or worried about a paper that's due, but that has nothing to do with my pile. I have a responsibility to look at every query independently of the results of that biopsy.
As for where you are on the pile, it actually helps no matter where you are. If you have a great query you'll always end up in the "maybe" pile, but if you have a so-so query that I'm on the edge about, it's actually better to be on the bottom (and why I'm shooting my mouth off about something you have no control of, I have no idea). Every once in a while I have a day with a big stack where just nothing is making it into the maybes. It happened today, but it was nowhere as bad as last week. Last week, for some reason, I was getting auto-reject after auto-reject. I couldn't put them into the maybe pile if I wanted to. Novels that were 20,000 words long. Novels that were 400,000 words long. Religious-themed novels. (We don't handle those) People who forgot to mention their novel in their long letter about how great a writer they were and how many writing credits they had (People do this). Stupid, stupid diet books.
After a while I get sort of desperate. Something's gotta be on that pile. I don't want to give my boss crap, but I also don't want to look too harsh. So I take whatever sounds remotely reasonable and maybe just has a bunch of strong previous creds (published articles in major literary journals, awards won, etc). It'll ultimately get rejected, but at least it's physically sitting there in the pile.
Anyway, that's my slice-of-life useless wisdom for today. As for why I don't post more, sometimes I'm having a slow week and there's not much to say and the questions are coming in slow. Or I've gotten enough personal death threats that I feel bummed. How Miss Snark manages to do multiple postings a day, I have no idea.