Confussion on roll-off factor, sampling time and Raised Cosine pulses.
An easy way to see what's going on is by plotting the signal's eye diagram.

With low roll-off factors, the eye diagram will show you that deviating a small amount from the optimum sampling time will decrease the error margin. Note that the signal peaks are not the ideal sampling points. This is the eye diagram of a sinc pulse (zero roll-off): 



With larger roll-off factors, the eye remains quite open for a larger period of time around the optimum sampling time. This is why it's said that these pulses are more tolerant of timing errors. This is the eye diagram when using pulses with roll-off equal to 1:



Mind you, this doesn't mean that you can be sloppy about timing synchronization. It's just that small errors in timing will result in larger variation in the sample values of low roll-off pulses compared to large roll-off pulses.