Well, that's a bit of a judgment call. Most people consider it to be a bit unsporting, or at least a tad cocky. But is it really worth reporting a player if they haven't said or done anything else in the entire game to warrant a punishment? For me personally, I don't report someone if that is the only thing they have said. There was a time when it really got under my skin, simply because of how unsportsmanlike it was. But now? I don't really feel bothered by the arrogance of someone who happened to do well in a single match of a video game. If they have said other things, or if they continue to trash talk in the post-game lobby, then yes, I will report them for it, but for just "gg ez," no, I don't.
And up until very recently, everyone was under the impression that players lowered the value of their reports by submitting "false" reports; i.e. reporting someone for something that they would not be found punishable for by the system. But in a very recent post by Riot Tantram, it appears that this is not the case. "Confirmed. Reports do not lose value based upon the number of times you report players. This used to be the case. It is no longer true." Source Thread I linked up a few comments so you can see where the discussion gets started, but basically, it used to be that way, where false reports would lower report value, but apparently that is no longer the case. My assumption, and it's just an assumption, is that since the IFS is much more efficient at reviewing reports than actual human man-power, it isn't quite as imperative for reports to be viable. It doesn't bog things down to have a few thousand false reports when the system can scan through them in a matter of minutes. It could take a human weeks to go through all those logs, only to have all that time wasted if they are non-viable.