I have never had a problem opening an earlier version document in 2007. It was designed to open any version of office documents or spreadsheets (automatically opens in compatability mode). I know there is a problem for some users opening office 2007 documents in an earlier version of office but this can be solved by always saving your documents in the compatible mode option.
If you are really concerned about your invoices etc, I would save them all on an external hard drive as backup (which is advisable in any case) and you will always have a copy to rely on.
There really is no issue with 2007 with regards to older documents and if anything is going to cause you issues it would be having two office versions loaded onto the one computer.