good advice.

when should I consider changing my plugs and/or wires?

my 2000 LTD has 67K.  it is a V8 and runs good.
You didn't say if it's the V6 or V8 engine. Ford's plug wires are pricey,
and for some reason more than twice as costly for the V8 than for the V6. A
Ford dealer will charge you over $200 for the 8-wire set. I would imagine
that they charge around $10 a piece for their super-ultra-sure-fire plugs.
Add the hour or so of labor, the coffee and donuts in the waiting room
(which will appear as 'shop supplies' or 'disposal fee' on the bill), your
local sales tax, and $475 becomes 'reasonable'.

The solution: buy your parts from an independent auto store. The same 8-wire
Motorcraft set costs $140 there and they have aftermarket for half  that
price (no clue about aftermarket quality, but my Motorcraft wires, purchased
from a Ford dealer failed in less than 30,000 miles, twice, so there is not
much to lose with aftermarket). They will sell you decent platinum plugs for
$3-$4 a piece if you don't fall for the quadruple flame tip nonsense. Your
trusted neighborhood mechanic should be able to buy parts and complete the
job for $250-$275, even if it's the V8 (you may need to get your coffee at
the Starbucks across the street, though).

Better yet, do the job yourself. Replacing plugs is not rocket science.
Access the ones on the passenger side through the wheel well. Make sure to
start them by hand, to avoid cross-threading. Apply a bit of anti-seize to
the threads and torque to the manufacturer's specs. No torque wrench? Buy
it - a decent one will cost less than an hour of labor at the dealership and
serve you for many years. Replace the wires one at a time, keeping the
original routing and avoiding mix-ups. Apply a bit of dielectric grease to
the contacts on both sides, to prevent moisture trouble.

Good Luck