The opinion (from a Scot) that stuck with me from a discussion in the uk.music.folk newsgroup was that no, put-on accents aren't good but yes, use the Scots words. I haven't quite worked out the general difference between put-on accent and attempted correct pronounciation. For example: 'doon' - is that an accent or simply pronouncing a Scots word correctly? If I sing 'down', I've anglicised it. If I sing 'clachan' and don't sing the Scots 'ch' and pronounce it as a 'k', am I mispronouncing a word or avoiding the fake accent?

I love many Scots songs and I avoid singing them because I haven't figured out how to do them justice and not sound silly trying.