When working with Google Maps and company one should keep in mind that Google Maps maps and orthophotos are shown in a standard Mercator projection (one of those mathematical solutions dealing with the ""Flat Earth" problem).
Often, routes build with the manual Basemap approach will by based on the UTM grid or on the O/S grid (UK). While closely related, standard Mercator and UTM or O/S grid are not the same. Ignoring this may lead to distortion and mismatch. The main reason for distortion is the varying scale of the standard Mercator projection, dependent on latitude.
Does it matter? It depends on the accuracy one wants to achieve. If DEM data is involved, precision becomes somewhat more important. One way around problems is to individually georeference each Basemap and work at large scale. This way errors do not accumulate and one would actually do the same as with Google Earth images.
To illustrate distortion, here is the "big picture" (small scale). The first image shows a map region as presented by Google Maps, MS Virtual Earth or Yahoo Maps.
The second image shows the same region converted to UTM. (Conversion to O/S would give similar results.) Distortion is apparent.
The third image shows a DEM overlay (11 SRTM tiles, the greenish area), also in UTM projection. Both DEM and map adhere to the same projection. If the didn't the DEM wouldn't match.
geophil