Summary: Use of an appropriately selected drill bit (see below) with a custom made tape handle works very well. Use of the same size drill each time and establishing a standardised technique (turns, pressure, ...) will allow a good reliability method to be established. This will be far quicker and more easily reliable than using any sort of knife, at the cost of slightly less compactness in extreme cases. Inspection of the finished cut is always wise to ensure that small whiskers of track do not extend along the hole edges to form a bridge - but consistent method will mean there will be very few bridges.
Detail: I used to do this often and my favourite method and tool is based on the experiences.
I found (as others have noted) that a properly sized hand held drill-bit worked well.
There is an optimum size range that gives best results - too small and you need too much depth into the board before you get a guaranteed cut across the whole track width, too large and it does not centre well and also tends to damage adjacent tracks. Actual size "somewhat to taste" depending on your style, but somewhat wider than track width. Try a few sizes and see what works best for you. AFAIR the best size was wider than the "proper" strip-board cutting tools.
I found that the "proper" tools tended to break off near the bottom of the handle because the metal shaft was not continued far enough up inside the handle and sideways forces would cause the plastic to shear. One may well ask why there should be sideways forces:-). Regardless of why, there were, and breakage was not uncommon.
When using a drill-bit, adding a handle at the held end can greatly improve usability and comfort. I found that a number of turns of masking tape worked well. This is the paper tape used by painters to allow production of accurate paint edges. It moulds and shapes well and can be squashed somewhat into shape as a handle.Normal operation is to rotate the bit against the board using thumb and index-finger while perhaps applying pressure against the bit end with the hand if needed. You can work out a standard number of turns and pressure which will reliably provide complete cutting but minimise board erosion. You can easily drill right through phenolic based board material if "over enthusiastic".
The use of a "spot face cutter" or drill bit has the disadvantage (seldom important in practice) of removing a complete soldering point from availability. Cutting between holes with a knife allows the extremely keen to use the immediately adjacent holes for soldering a component leg BUT if you need layout that tight you are going to need extreme care overall to avoid solder bridges.