I often forget to take the service booklet to the dealer.
The receipts are more important imo as they prove what you've changed.
I also write on the back of the service slips, to record all the bits like oil changes, chain & sprockets, etc, but this is more for my benefit than anyone elses.
Anyone can get a stamp put in the book for the cost of a pint to the right person, and for me the appearance of the bike generally shows how well its been looked after.
If the service book was missing though I'd be suspicious