I generally don't do huge rebates, and have never bothered using certified/registered mail for sending in the rebates. I did have a $150 rebate on my laptop, and set it regular, and it went through without a problem.
I'd have to say the most important thing would be to make certain you make copies of everything. I can't tell you how many times I've received notification that I'd sent the wrong documentation (missing upc, etc), called the rebate house, had the CSR there tell me the same thing (usually in their haughty "aren't you a dumbass??" tone of voice), then gotten it approved when I told them that I not only had photocopies of everything I'd sent in, but that I had a photocopy of the upc I'd cut from the box, stapled to their rebate form. Sorta shuts 'em down real good, ya know?
