About to change out the old headlight assy's on my 04' F150. I bought new headlight bulbs for the new assy. The new assy comes with already installed side marker and blinker bulbs. Question is..should I put a small amt. of dielectric grease inside the plug-ins or not?

yes

Great idea! You're the first one here I have ever seen mention even knowing that such a compound existed!
Put a dab on the bottom side of the terminals that go onto your boat's battery post while you've got it on your finger.....

Thanx....now to go get some!! Ya know...Every U Tube video I have looked at regarding changing out the whole assy's and bulbs have not one mentioned using this grease during installation.

LOL, I was parts man at a fairly large fleet automotive shop with 10 mechanics and it was hard to keep enough of that stuff in stock! They all knew the preventative maintenance value of using it on all electrical connections...

yep, I keep in my tool box. I use it on battery posts, wire connectors with possible moisture exposure and wire nuts in septic tank.

while you are at it have some led bulbs put in .I had some BAD AZZ led bulbs installed in my 02 $ 80.00 Glad I did

please don't put LEDs in halogen housings. You will blind oncoming traffic. If you do be careful what bulbs you choose. There are some newer bulbs designed to work in halogen housings. They were not cheap but when I swapped mine I went with some retro fit Profile LED projectors. Huge upgrade over stock halogens. I also put LEDs in fogs. Now my fogs actually light up the road directly in front of the truck.