If you go to http://www.scribd.com/Lew%20Merrick
, you will find a document Pilot Hole Considerations
that gives the equations for calculating different fits of pilot (tap drill) holes and their impact on the strength of the joint. The "standard" tap drill is supposed to create a 75% of full-thread engagement threaded hole. This is a convention based on providing maximum tap life in most instances along with generally acceptable axial load carrying capacity in the joint. If an actual 75% of full thread engagement is used, then the joint is 90% as strong as a 100% full thread engagement joint.
The standard M5 thread is more formally defined as M5 X 0.8mm pitch thread. A ø4.5 mm pilot hole in this instance would create a 48% of full thread engagement joint. This would be joint that is 53% as strong as a 100% full thread engagement joint in axial loading. That would be the best
you could get from the joint as other factors (quality of the tap, quality of the tapping set-up, etc.) would degrade the value from there.
Yes, 90+% of the time using an oversized (by the chart) tap drill will work just fine. The problem is that a real
answer requires an understanding of the application and loading of the joint. The thing working in your favor is that most designs use screw/bolt sizes significantly greater than actually required. A typical 1100 MPa yield strength M5 screw will not yield until 13.8 kN of force is applied to it. Designers often use "eyeball sizing" rather than looking at the actual loads. This leads to conditions of (1) using a much larger thread than necessary; or (2) failing the thread (usually when the screw or bolt goes into bending mode).