Greg doesn't give you "hints"- he just gives you information about the evidence. You want the evidence to have red tags on them- when you first pick it up, they are untagged, Greg will tag them and once you have figured out everything to do with that piece of evidence, the tag will go red. Therefore, annoy Greg all you want
You can only ask hints from the CSI you are working with, ie. Warrick, Grissom, Sara, Catherine or Nick. These are the "hints" or questions that will impact your case score.