reading an EEG dataset using EEGLAB
If I understand your question correctly, yes, since ERP involves averaging over similar events, very accurate time-locking to stimulus onset is critical.  The idea of an ERP is that activity relevant to the stimulus will happen at a similar timepoint after similar stimuli--if you don't have an accurate time stamp (usually to the level of single milliseconds if not better), then the activity will be "smeared" across the time course.  For a good overview of ERPs, see the article Event-related Potentials: An Overview