The stock engine needs 87 octane (we use R+M/2) at sea level and the higher you go....the less octane you need because of the reduction in atmospheric pressure. Many States here that exist at altitudes of 4000+feet only have 91 octane as their highest where we down here near Sea Level have 93-94 common at the pumps.
Your 98 is very likely much the same as our 93 rating and unless you've modified the motor pretty decently it's not needed even at Sea Level. If the bike seems to like the 95...by all means use it!
The stock SV motor isn't hard on the octane and detonation shouldn't be a problem.