It would be a good idea to keep all those details. I do keep a record of what i pay how much i spend so if i ever have to sell i remember what they owe me.
I am not sure if I want to keep track of how much $$ I have spent on this hobby 
Me either
id probably get scared at the amount i spend on each machine i restore!! though i'll be its not as much as some of you guys spend on repairing/restoring your games.
Having said that, i HAVE kept receipts and parts invoices for most of the things i've bought over the years for each game.. i just havent added them up for the reason mentioned above
Its an interesting idea though i must say.
