The importance of a well levelled print bed can not be emphasised enough. The lower the layer height you print with the more critical having a perfectly level print bed becomes for trouble free adhesion of the first layer. The use of a Raft or even altered first layer width or thickness is often a compromise for imperfect bed levels or surface flatness. This is functionality accommodated within slicing packages, e.g. Slic3r. I've never had much success with rafts as they have always proven difficult to detach from the printed object. Rafts are commonly used in commercial printers and also successful on the UP! printer, but the secrets of good raft separation still seems to elude the OpenSource community, it being theorised to be perhaps both material and software related as discussed here by Nophead, although Chris' desire to get to the bottom of this does extend to having good peel-away support, not just raft.
I think, in printers where the print bed is typically un-clipped to remove the finished work, the precise levelling of the print bed can't always be guaranteed so rafting is a good compromise. If you don't want to use rafts or alter the first layer width/thickness then a very level print bed is essential for good contact of the first layer.
I opted to use the Helios Heated Print Bed (HPB) in my scratch-build Mendel90, instead of the more popular Prusa HPB (typically red). I have a Prusa HPB already so its interesting to compare.
My initial expectation was that the Helios would be even more suited to 3 point support and the inherent ease of levelling that brings. The Helios board is twice as thick as the Prusa board. However, I think my expectations have risen over time, and in borrowing a dial gauge to level the new M90 Print Bed I was disappointed by not being able to get it as level as I'd hoped with just three points of support. The problem came down to a slight warp in the HPB, which can't really be described as a fault, as the the deviation is only about .2mm along one edge that was only supported in the middle.
Perhaps if I used thicker glass then the clamps would pull the board straight and leveling would be fine with three point levelling, but I am happy with my light glass (1.3mm thick), as it keeps the overall print bed weight and inertia down.
The Helios comes with long M3 counter sunk screws, some washers, spring washers and nuts. I like the ease of adjustment that spring loading the bed brings, but you have to use strong springs here to avoid wobble.
Photo showing counter sink screw, washers and bolts that come with the Helios board. I use a strong spring (also shown). The Helios fittings are stainless steel which conducts less heat, a good think in this situation.
An essential prerequisite is that the x-axis has been levelled with respect to the printer base. This can be done with a vernier callipers (see photo above), moving the callipers from left to right and turning the Z leadscrews independently of each other, until the x assembly is perfectly parallel to the base.
To hold the dial gauge to the x-carriage I printed off Nopheads dial gauge holder. If your dial gauge has a narrower (8mm shaft) this other clamp may fit better (http://www.thingiverse.com/thing:92852).
You don't have to have a dial gauge to level the bed but it's more time efficient if you have access to one. You can level the bed by using a sheet of paper or feeler-gauge to set the gap under the nozzle at various locations around the bed, and adjusting the bed height until the gap is consistent.
I did some video recording while adjusting the bed level, and generally checking the level after adjustment. I'm sharing an edited down version of my recordings below. It is not intended as an instructional video so don't view it as such. In the first section you will see me adjust the nuts to raise or lower the bed. There is only .1mm between each of the visible numbers on the dial gauge, and the smaller incriments are just .01mm, but what I concluded by the end of the first section was that I was not going to level the bed on the right hand edge. The second section of the video shows how well I was able to level the bed with four adjustment points (each corner). If you observe the movement in the dial gauge as it moves around you will see that the bed is levelled to within a very tight range, estimated +/- .02mm!
4 adjustable points of support will (most likely) be better than 3 if the print bed top layer consists of the HPB board and thin glass.
3 points of adjustment is easier to level, but the board or glass must be rigid and flat.
A dial gauge makes bed levelling easier and quicker.
A well levelled print bed enables consistent printing of first layers, giving good print quality and reduced risk of print warping or detachment during printing.
Hope it was of interest!