How do I interpret an effect of electrode position in ERP research? The example I have in my research is that both conditions show greater amplitude and latency in the parietal electrode sites than in the occipital ones.

I'm not sure how meaningful this is for my research question, which is primarily concerned with how processing of the two conditions potentially differs (spoiler: no effects of condition!). The effect of electrode sites does not interact with anything, so is just the case that there is greater activity in the parietal sites - can I argue something about the processing involved in both conditions based on this finding?
The question is quite vague, as it is unclear what the sample size is and whether OP is talking about an anecdotal experiment or the outcome of a well-powered statistical analysis. Moreover, there is mention of two conditions, but I fail to see what the difference is between the two.  

Assuming a normal sample size, I can say that the measured amplitudes of electrophysiological signals are often unreliable. This is due to a variety of factors. 


Impedances affect signal size, as high impedances reduce the measured potentials. Impedances can vary between subjects, but also between sessions within subjects, due to for example differences in the effectiveness of the scrubbing and whether enough conductive paste is used in case of EEG. 
Impedance differences between active and reference electrode affect noise level. Noise may vary from day to day and even within a session due to interfering background electric fields.


In contrast to amplitude, peak latencies are often far more reliable. 

So it all depends on the power of your analysis. If there is truly a difference in amplitude, then you should look at the physiology involved in your task and see whether brain mechanisms can be involved.