Imho, brackets like these are never 'useless', maybe they're just not mandatory in this specific situation.

But (at least in most situations) they increase readability, thus I think it's a good idea to set them habitually, uniformly and univocally - and one won't run across problems in other situations where they might be mandatory.

besides these:
on *:JOIN:#ChanName:if $nick == MyNick mode $chan +o $nick
on *:JOIN:#ChanName: { if $nick == MyNick mode $chan +o $nick }

The following are all "common styles", too:
on *:JOIN:#ChanName: if $nick == MyNick { mode $chan +o $nick }
on *:JOIN:#ChanName: if ($nick == MyNick) mode $chan +o $nick
on *:JOIN:#ChanName: if ($nick == MyNick) { mode $chan +o $nick }

on *:JOIN:#ChanName: {
if ($nick == MyNick) { mode $chan +o $nick }
}

... and I do prefer the last two styles, for reasons named above smile