The default Snap distance setting in REAPER is 4 pixels. I always change this setting to a greater number for the sort of work I do.
Select
Options > Preferences > Editing to access the
Editing Preferences page:
http://documentwrite.com/reaper/index.html?editing2.htm
The setting to adjust is "Snap distance 4 pixels". For most of my work I change this setting to 50 pixels. YMMV.
Note that the snap setting is Zoom dependent, so the further you zoom in the less you will notice the Snap behaviour, and the further you zoom out the more you will notice the behaviour.
One day there will be an option to make the "Snap to Grid" behaviour independent of Zoom. This will be a good thing.