Upgrading to Exchange 2003

Ok, got a new server in and attempting to install Exchange 2003. I currently have a Windows 2000 Server with Exchange 2000 on it.

Going into it, it asks to run domain prep and forest prep. Is it ok to run these during business hours? I have always heard bad things of running these on the clock. But I cant understand why!

Could someone let me know if this is a good idea to run during business hours, or just wait for a weekend or late night?

Thanks!