When submitting to agents I came across many who wanted either the first five, or first ten pages of my manuscript. Fantastic, I thought! Someone who will read at least a bit of my writing before immediately rejecting me! And it is great, if you're prepared. I wasn't and didn't know it. I began submitting immediately, at great postal expense I might add.
After many rejections I finally printed out my manuscript and took a red pen to it. I was appalled at how many errors I found and embarrassed that I had sent it out in such condition. But I had gone over and over it on my computer, I couldn't believe I had missed all those errors. Well I had.
For many of you, reading and editing on your computer will be the way to go. For myself, it was printing it out and taking a red pen to it. Either way is great, you just need to figure out which way works best for you and do it. If you have a friend you trust that will help, by all means recruit them.
Never send your work out without extensive editing on your part. That said, don't just apply this to the first five or ten pages that you're submitting. Be confident that they will ask for the whole thing and do it all in preperation.
Must one use a copy-editor, I wondered? If you aren't decent at editing, or can't be unbiased towards your own work, it wouldn't hurt. Do you have to? No, definately not. It depends on your level of competency when editing, and of course on your finances. Copy-editors are expensive, ecspecially when you write everything over 120,000 words like myself. So give it a go, you might find that you're your own best editor. To a point of course! At any rate, you'll end up with a much better manuscript that reads like you put a lot of work into it.