I'd do it now. I have a 2006 with about 50K miles and have changed them twice. I Kroil penetrating oil, let it seep down for a good hour before removing them, never had a problem this way.

Caution: Do not remove plugs when the engine is warm or hot. The engine must be at room temperature when performing spark plug service. Removing the spark plugs from a warm/hot engine increases the chance the threads could be damaged.

1. Remove the coil-on-plug assemblies and thoroughly blow out the spark plug wells and surrounding valve cover area with compressed air.

2. Back out the spark plugs no more than 1/8 to 1/4 of a turn. Using Motorcraft Carburetor Tune-Up Cleaner, fill the spark plug well just above where the jamb nut hex sits (1/2 – 3/4 teaspoon). A minimum period of 15 minutes of soak time is required. The cleaner will wick down to the ground electrode shield and soften the carbon deposits in this time. Do not work the spark plug back and forth at this point.

Caution: Excessive Motorcraft carburetor tune-up cleaner, or repeating the process several times with too much cleaner fluid, could introduce enough liquid volume to hydro-lock the engine.

3. Tighten, and then loosen the spark plug, working the plug back and forth. Some screeching and high effort may be noticed. The expected removal torque is about 33 lb.-ft. (45 Nm). Repeat the back and forth turning as needed until turning effort is reduced, and remove the spark plugs.

Do not use power tools for the plug removal — spark plugs must be only be removed with hand tools.​

 

I work out of my home, and I'm lucky if I drive at all most weeks outside of taking my son to boy scouts..... and I drive my F250 for that stuff.

 

The problem is the 2004-early 2007 5.4L 3v plugs are a two piece design with a gap between the wall of the bottom of the plug and the cylinder head. Carbon builds up there, seizing the bottom half of the plug so when you turn it out the top half snaps off. There are more topics about this design flaw over the years than I can count, so many people ending up with either the bottom half stuck in the head or dropping into the cylinder.

It got bad enough that Ford made a tool for retrieving the bottom half, and ended up settling a class action lawsuit
in 2015.

Here's a guy who had 7 of 8 break off, you can see bad carbon build up on the bottom (and it looks like one of the plugs left the bottom piece in the head):
#ad


And a picture of what this plug looks like in the head. They don't show the gap in the picture, but it's between the part that says seized shell and the cylinder wall. There's no way to prevent the build up, so now people either buy the one piece plugs Champion makes (not an option for me since they don't have a colder rated plug for my supercharger) or they change them more often before there's much build up.
#ad

 

Yeah, not one of Ford's better ideas.

 

I prefer Kroil, it seeps better in my experience.