based on my experience using Uber and Grab, you can drop the pin exactly at your current location

Best thing is if you are unsure, just message the driver and let him know your landmark just after you booked . most driver can accommodate your request as long it is reasonable. 

if I am at unknown place, I will open Google Maps first. zoom in and identify your current location then you can open your grab/uber apps and key in the location stated in the Google Maps.  just don't panic.

but yes, you can panic when your hp failed to get a good GPS signal. in that case call the driver lol 

nope. do not pin point on road. always use a landmark. a shop like 7-eleven, mamak restaurant, etc. if you put on road will be Jalan Klang Lama for example. the whole stretch of road is Jalan Klang Lama.

for shopping mall example KLCC, just put Coach@KLCC or Mid Valley put Coffee Bean so that driver will know which entrance to go.

most drivers will doubt if receive a request along a long stretch of road. because normally always bangla the main culprit who simply drop their point. I once received a request up on Akleh highway the point. of course I will not accept such crazy request.

so to minimise the doubt. use landmark. it is the safest.