There are two many schools of thought regarding daily carry knives.
The first believes that a dedicated defensive knife is the way to go, and this knife should only ever be used for defense.
The second school of thought believes you are likely to use a knife even better if you actually use it on a daily basis, instead of just carrying it.
School #1 doesn't require the knife to be sharpened frequently, since it is only ever used in emergencies or after practicing defensively (because it would just be stupid to carry a defensive tool that one never practiced with, right?). The user will need to carry at least two knives, one for tasks, and the other for defense. There are potentially more legal consequences to this approach, since the dedicated defensive knife frequently looks like it exists to cut people.
School #2 doesn't require carrying more than one knife. The knife must also be a good shape for daily use, so is generally likely to be viewed as more acceptable if seen by ordinary citizens or law enforcement. It does require more frequent sharpening and knife maintenance.
In both cases, the knife in question should be easily carried and concealed and (of course!) legal where carried. This automatically means any knife with a blade over 4.5" blade is out. Any larger knife is just too cumbersome to carry on a daily basis.
John