Unless you are VAT reg, the tax man doesn't need copies of your invoices, in 25 years and 2 investigations I've never needed to supply them.
I know they do if you are vat reg though.
Quite apart from which, if a domestic customer has been in I've never given one out!
And this is MBS' first commercial contract, ergo his others are all residential (presumably??)
But I do agree with Alex, keep them safe as they can be referred to if and when you have problems with payments in the future.
Ian