Don't put any color LEDs in the backbox, that looks like trash.
Don't put any superbright LEDs in the backbox, that looks like trash (and makes more glare on the playfield glass).
If they game has heavy Yellow tones (like a Comet), you might have to just stick with incandescents, as even Warm White LEDs bleach the backbox artwork.
Sometimes Warm LEDs look good if you skip some of the backbox holes so it's not so bright. Just like your girlfriend - you don't have to fill every hole.
Color match all the inserts with single SMD or even dimmer LEDs. DO NOT put superbright LEDs under the inserts.
Color match all the Star Rollovers with "bendies" LEDs with the flexible leads. You can also use bendies to light saucers and chutes.
You can probably skip buying any Yellow or Orange LEDs, they are pretty weak - use a Warm White instead and the inserts will look terrific.
Strips of LEDs are good to run along the playfield rails and illuminate dark areas. Under ramps is often good too.
If you add strips or any other oddball lighting, make sure the player CAN NOT see the LEDs themselves. All light should be reflective or backlighting.
You can get shiny metal ALUMINUM TAPE (not duct tape) in the insulation isle of Menards. You can use this on wood rails under plastics to help reflect more light towards the ball. It's easily removed.
You can paint the backsides of black ramps with white paint to make them reflective - think of Space Shuttle as a great example.
Do not put any colored LEDs in the GI (unless it's Fireball Classic or Space Station where it came from the factory like that).
Read the above again ^.
Buy extra LEDs of each color. Some will be duds, some will have the heads break right off.
If you can't get a #44 LED to turn in the socket, rub the nipple across the concrete to shorten it. If it's too long, it prevents the bulb from latching.
Never by premade "kits" for a title, it's cheaper and better looking to DIY.
When in doubt, check here for things never to do to a pinball machine: