Very safe IMHO. I live here and have no issues about leaving my bike overnight anywhere without security devices attached other than the OEM bar lock (which are rubbish anyway).

I may give known night time hotspots a miss though. For example there are two Douglas nightclubs (Courthouse & Outback) which are idiot magnets. I'd avoid leaving it around them. In Peel, I'd probably avoid the Creek Inn area. This is not because of risk of theft but simply because of the large crowds and/or drunk people and the possibility of accidental damage.