That's not true, I've checked with my (three) teachers. It is the right kanji, but it's not used for this boku in written Japanese any more. It's a throwback to the time when all personal pronouns were really adaptations of other words, in this case "servant". The reason it was considered masculine is the apparent machismo of serving a great samurai lord.
In that sense, it is the right kanji, but you shouldn't learn/write it. I just keep it though because I can't be bothered reverting to kana.
As for kanji use, yes, there are no "bonus points", but you do give the marker a better impression if you can use kanji well. Our teacher has asked a previous (and possibly current) senior marker to go over our work, and was told quite plainly that the better responses often mixed extracurricular kanji and grammatical constructions into the fold.
Basically, it doesn't matter that they're not on the syllabus. What matters is whether you use them correctly, and use them well.