The way the SNMP counters work with MRTG (rrd) is that they are the total traffic sent. They than wrap around at 2^32 and 2^64. Then MRTG takes the measurements and figures out the rate from the difference of the two measurements. So it is not like it is taking a snapshot of the current rate.
So my point is that if you poll for the amount of traffic sent more frequently you are not actually going to get more accuracy but rather a higher resolution in time. You will be able to see what the rate was during a specific minute as opposed to every 5 minutes. So when you poll every 5 minutes you are really getting rate per 5 minutes converted to a rate per second such as megabits per second.
You also might have to adjust the rrd files so they don't do averaging over 5 minutes regardless of how many samples you take.
That being said, you can probably poll every minute from SNMP, just make sure you monitor the CPU.