Hi Dale,
There are many factors to be taken into account when deciding whether or not to purchase certificates. Some of this I suppose comes down to gut reaction which is not a reliable method

. These are the main ones I use to try to get out of buying certificates.
The ancestor with the unusual name can be a double edged sword - easy to find or lost through mistranscription or misspellings or spellings not as expected today. I've been lucky enough to have a couple in my tree. These are especially useful pre 1837 as often there's little else to go on.
I like census too - mother in law in the house on census night can save £7.
Australian death certificates are helpful. I spent ages looking for the right wife in England, only to find her (or by this time, have it confirmed) by a son's death with the mother's surname in the index.
You do have to be careful though, and I've got a little off track and confused a couple of times, but sorted it out through purchasing the certificate.
I've only ever bought one English death certificate.
I'm sure others will have more suggestions.
And to get back on topic ... I've been thinking about the celebs for WDYTYA series 2. Wonder if we should start a list? My first guess is Hugh Jackman.