I have a friend ( an agent) that I lovingly call the Typo Man. He got this nickname because he has this amazing ( and annoying) talent for picking out every single typographical error in every manuscript he looks at.
Now, I spend a great deal of my time editing and fixing manuscripts but I have to say that I have nothing on the Typo Man. If I thought I had some skills, he puts me to shame. But that's a good thing because when I need someone to edit my work he is always there to fix up my work and put his stamp of approval on it.
As agents, our aim is get your manuscripts through the front doors and get published. You don't get them published with typos. Even the smallest one can mean your book might end up in the garbage can. Of course you could get lucky. There are plenty of people who have published manuscripts chock full of typos, but the work was brilliant enough to get through the dismal spelling or punctuation errors.
However, with publishing houses cutting back on editors every day it's so important to take the time to do more than spell check and have your friends read your manuscript. You NEED to print your work out ( Stats say it takes you 25% longer to proofread on the screen), read it out loud, and send it to someone who can either edit or give you an honest opinion. And I'm sure the Typo man will be reading this as well. I'll post any errors that he finds!