No its not obvious. The SNMP byte counters are odometers - as long as you get two clean samples per counter wrap you can accurately count bytes. The trick is to ensure that you get a minimum of two clean samples of the odometer reading per counter wrap - for high speed interfaces that typically implies reading the MIB2 64 bit interface counters, or triggering an SNMP poll at relatively tight time intervals.
FYI: 2^64 is 1.844 * 10^19 at 2.5 gb/s (OC48 line speed) (2,500,000,000 bits/sec), you transfer 312,500,000 bytes/sec, or 298 megabytes/sec. 2^64 / 312,500,000 = 6.189 * 10^16 seconds per rollover. or, 1.719 * 10^13 hours or, 1,962,741,057 years. My point: at least for the near future, 64 bit counters won't roll.
(My previous comments a month or so back about the inaccuracies inherant in 95% systems still apply - given a particular (extreme case) traffic load pattern it is possible for two measurement systems that are not phase locked, using precisely the same sampling technique and computation to deliver outcome values for the 95% point where one is up to twice the value of the other. )
Of course; but, if using 64 bit counters, they should be damn near close.