[snip]

Comparison shop.

Go to two or three different dentists (in totally different clinics) and
have each one evaluate you and make recommendations.  In each case,
pretend that you're only seeing that dentist.  Also, don't "lead" the
dentists by suggesting anything - let them all draw their own
conclusions without any prodding from you.  When you're done, sit down
and compare the recommendations from all the dentists; wherever there's
overlap, that's probably something you ought to have done.  Then go back
to whichever dentist you liked the most, and make an appointment to have
the work done.

I'm in my 40's and have quite a few heavy fillings in most of my molars. I
recently attended a dental clinic for the first time and they advised me
that I need 6 crowns. Apparently if I don't get the crowns I run the risk of
breaking the fillings and the underlying tooth. Just how true is this, and
do crowns last longer than fillings? I asked about the longevity of crowns
and one dentist said they could do me my lifetime, whereas another one (in
the same clinic!) said 10 - 15 years. Asuming I live longer than 15 years,
if I get 15 years from a crown can I simply get it recrowned?

One last unconnected question... is cleaning of teeth by a dentist a
necessary procedure? This clinic have advised that they clean my teeth, even
though they have inspected them and classed them as being in fair condition.
It has been about 3 years since my last cleaning. What gives me some concern
is that they want CAN$150 for the cleaning.

I guess dentists are just like any other business. As soon as you walk in
the door they try and sell you all sorts of stuff. Some worthwhile, some
not. The problem is knowing which is which.