you really have to make sure when you're doing this it is by an Apple Certified Company or else your warrant is void I believe.
That's only if they can tell it was messed with. Places that put screws back in halfway or lose half of them make it obvious. I see tons of stuff done like that. It's not a byproduct of incompetence, rather a result of working on a machine than waiting to put it back together at a later date. It's easy to forget where things go when you work on 20 different machines a day and come back to one 2 weeks later.
Wherever you ago, make sure they have some sort clean room, whether self-made or professionally done. Apple authorized or not isn't the issue. Dust behind the glass is the issue. The glass can be replaced without doing the whole assembly but most places doing it, even the places with the spiffy logic board component level repair knowledge do it in a regular dusty room and do nothing to ensure the glass doesn't have dust on the other side. Typical side effects of this are little rainbow-effected spots where you see separate individual colors on a pixel because the light is being refracted off the dust particle, and can drive you nuts if you spot it.
I can attest this was a pain in the ass, fixed by nothing less than getting 5-7 air filters put in a tiny room and keeping them on for a day before attempting to do any repair of the kind. Even then, dust on the clothing and whatnot seeps behind there.
For DIY using household materials I always recommend using a dehumidifier and your bathroom, as it is a common household item that typically contains a good air filter. A smaller room like a bathroom will be easier for a tiny air filter to clean all the dust out of.
Before the air filters I'd spend 45 minutes getting every last piece of grime with a CD wipe in a semi-clean room, and an associate would not even bother and the customer didn't notice any of the dust or bits behind the screen. Someone who will remain unnamed who does apple products in the east village leaves eyelashes behind the glass and people don't notice.
So maybe it isn't as big a deal as I make it up to be.
If it is just a chip in the corner and not a total crack, unless you require perfection, I wouldn't bother. If it's a crack in the viewable area, I would go for it. Chips don't always mean there is a definitive crack that is likely to spread to the rest of the glass, but if it is a direct crack and is in the process of spreading I would get it replaced as micro fractures will only ensure it gets worse over time.