I'll share some info from firsthand experience here. Bum arrest, bogus charges, and pre-trial intervention (like probation but charges dropped after completion, and that was an ordeal in itself) because a jury will listen to a ranting vice principle before the long-haired teenager.
An arrest will show up and cause problems more than three years down the road, even with charges dropped and after the court tells you it will be cleared. It won't, and you can safely assume everyone involved from that point on is feeding you bovine excrement.
Get it expunged. Even do the paperwork yourself, but get a lawyer--this is affordable, because it's just following a paper trail and yelling at everyone from point A to point B, to C, to D, back to B, to 27, back to D, etc. Doing the paperwork myself got me half price, but I had to get a lawyer.
Fight tooth and nail the whole way--the only way to get this taken care of is to make it easier to just sign the paper (seriously, it's just signatures!) than shove it off and forget about it. I actually had to tell the DA to her face that someone wasn't doing her job and note that I could have my defense lawyer look into it, and there was no good blood between the two of them.
And, by law (in FL, at least--IANAL) all agencies must destroy the paper trail, to be reopened ONLY by court order. Some law enforcement agencies still like to pretend the law doesn't apply to them. Aggressive lawyers are good.
Ahem. Yeah, I'm a little bitter. Excuse my ranting.
TL;DR version: search out a lawyer. A couple hours of work, a firm hand, a relatively small check to a lawyer, and a couple follow-up calls and it's taken care of. You may have to make a couple calls afterward, but I let my lawyer know I would only settle for dropping off the radar or enough heads on pikes to keep bands of highwaymen out of my town.
And keep receipts--it's still a lawyer.
But, all told, expungement took about a year--including the 6-8 monthes of the DA going "It's not here yet. It's here, waiting to be signed. Signed, waiting to be sent. Unsigned, unsent. It's signed and sent. Wait, it's unsigned, I'll get on that. We sent it last week--wait, it's right here...
But a week after I got my final paperwork, I bought my first pistol. No trouble any more, background checks think I'm a saint, and--as said in the final paperwork and the judge's own writing, unless I'm under oath or court order, I can legally deny ever laying eyes on the back seat of a Crown Vic.
(Disclaimer: IANAL--check your laws)