My personal rule of thumb that the PathParam leads upto the entity type that you are requesting.
/Invoices // all invoices
/Invoices?after=2011 // a filter on all invoices
/Invoices/52 // by 52
/Invoices/52/Items // all items on invoice 52
/Invoices/52/Items/1 // Item 1 from invoice 52
/Companies/{company}/Invoices?sort=Date
/Companies/{company}/Invoices/{invoiceNo} // assuming that the invoice only unq by company?
To quote Mr Rowe: Path parameters for grouping data, query parameters for filtering