There are plenty of commmon necron questions - wraith loadouts is one of them.
The one that I see a lot of is "do I put guns on my wraiths?" I'll answer that, and I'll also go into the melee loadout while I'm at it.
Guns? No*. But why?
Wraiths are 35pts base, regardless of melee options. You can pay 5pts to pick up a particle caster (S6, AP0, 1 damage, 12" Pistol 2) or pay 10pts for a Transdimensional Beamer (S4, AP-3, 3 damage, 12", Assault 1). The particle caster is some decent incidental shooting, and the beamer will knock a hole in something.
So, why are we leaving them at home? Four reasons reasons.
1) BS4+ - wraiths just aren't that accurate. A full squad of 5 (you can take six, but that gets ugly for blast) hits with 5 particle caster shots, or 2-3 beamers. You're paying 25-50 points for that.
2) Charge range - if you're about to charge, and want to soften up your target, remember that dead models increase charge distance. Remember that you paid to do this.
3) Wraith Form - you move through terrain without penalty. This, on top of your T5/3W/3+/4++ defensive profile, means that you can avoid exposing yourself to fire, period. At 3W, you're relying on your statline more than Reanimation Protocols to not die.
4) How Often You'll Shoot - You might get one turn of shooting. So, the ideal scenario for wraiths is to sneak up on a target, then charge through terrain and tag it. If you survive the enemy's turn after that, you should be trying to engage something else and possibly pushing deeper into enemy lines. You can fall back and charge. Anyone that knows what Wraiths can do is going to prioritize killing them once they're exposed.
*if you're paying power level, knock yourself out with a gun since points don't matter.
But X makes guns good!
I have seen a few arguments for bringing guns on Wraiths. I don't really buy them. It boils down to the fact you're paying points for crappy shooting.
"But shooting can do damage" - Yep. Absolutely can. Wraiths aren't reliable at it.
"But I can shoot those pistols in assault!" - You can, but how often are you going to do it? Shooting pistols in assault matters when you're stuck in assault. Wraiths don't HAVE to be stuck in assault. Unless you're hitting something that's hidden to everything but the wraiths, you're probably better off falling back, shooting at it with other stuff, and then seeing if it's alive to worry about.
"But the beamer does 3 damage and that's neat" - It does, if it hits and wounds. A given beamer has a 20% chance to kill a primaris marine, between BS4+ and S4. It goes up to a little over 50% if you spend a CP on the Techno-Oracular targeting stratagem. I seriously think you can find better places to spend that CP.
"But I can get to BS3+ with a technomancer and canoptic control node!" - Absolutely an option. This is neat if you're doing power level, I suppose. However, the real fun/mean way to do that is bring a node and a failsafe overcharger to make those Wraiths go from WS4+ and 4 attacks base to WS3+ and 5 attacks base. That's just bloody hilarious. And also doesn't really care about the guns.
Opportunity cost is a $20 term for saying "you brought X instead of Y." Or, you should compare what you're giving up to get a thing. Assuming we're paying points, you're looking at 25-50pts for the guns, or the following -
2-4 Necron Warriors
1/3 to most of a Lokhost Heavy Destroyer
~1 to ~2 Lychguard
~1 to ~2 Praetorians
1-3 Scarab bases
Basically, you can fill out other units, or start paying towards something beefier. Wraiths are unique in that they're a tar pit / potentially killy unit with obscene mobility, but lean into their strengths.
A Note on Scarab Melee Options
While we're here - the melee options are free, and there are two of them. You can take Vicious Claws, or Whip Coils. Claws mean you swing 4 times at S6, AP-2, 2 damage. Whip Coils mean you'll swing 8 times at S4, AP-1, 1 damage.
So, what do you pick?
I lean towards the claws. Why? The Necron codex doesn't have a lot of decent multi-damage weapon options (unless you want to drop 1d3 or 1d6, you're looking at Praetorians or assault Destroyer types). Claws excel at stabbing Marines to death. They can also do a number on vehicles as well.
This isn't to say Whip Coils are bad - it's just that the Necron book has plenty of options for a volume of low-AP anti-infantry attacks. Whip Coils also are liable to bounce against targets with decent saves or higher toughness.