Unlike other printing technologies (SLA), printing on rafts is not needed or even desirable in most circumstances. Printing directly on the build platform is the way to go for surfaces which are flat.
One thing to consider though if dimensional accuracy is important. The first few layers, especially the first one are heavily compressed in order to "press" the plastic into the built platform for adherence, so you'll find that the bottom layer will squish outwards, leaving a bit of a lip on that edge, while at the same time being thinner than what it should be.
The amount of deviation is dependent on a lot of things including the actual gap between your nozzle and the print surface, the flow, speed, etc. Simplest way to find out is to print a test piece, usually a cube of known dimensions, and measuring it after wards. This way, you can modify the piece you're printing in the CAD program to account for that deviation.
To print or, 3D print, that is the question...
SD3 printer w/too many mods, Printrbot Simple Maker Ed., FormLabs Form 1+
AnyCubic Photon, Shining 3D EinScan-S & Atlas 3D scanners...
...and too much time on my hands.
