The precision one can achieve depends pretty much on the quality of the plot and of course on how careful the scan is performed. For a high quality plot and a carefully conducted scan the precision is realistically not better than the equivalent of approximately one pixel. To get a feeling on the achievable precision select View->Current Precision. The information dialog that pops up displays the current precision corresponding to +/- one pixel in every direction. For logarithmic axis the errors will be of course asymmetric. Although not a perfect estimate, the errors shown should give you a feeling on the precision of the scan and how serious you should take the obtained values. This of course not applicable for distorted plots.
Note: zooming in on an already pixelated plot will typically not increase precision.