I think there's about 4 levels of tax "minimising" (am trying not to use a technical term).
Level 1: Using schemes that have been deliberately set up by the govt to allow people to pay less tax. For example ISAs, pensions, VCTs. Nothing morally wrong with taking advantage of these schemes, as that is exactly what the intention was behind them. Not illegal either obviously.
Level 2: Arranging your life in a way that you otherwise would not choose to do, simply because it is more tax-efficient. For example, Xenia's hypothetical couple who choose to each work part time rather than one full time and one SAH if they have only made that choice because of the tax consequences (i.e. they would actually prefer to have one SAH and one F/T if the financial effects were the same). I don't think this is morally wrong but I do think that the tax system should aim to avoid these sorts of "unintended consequences" wherever possible - so it should be tax neutral whether you have two P/T spouses or one F/T and one SAH. (Tricky, I know).
Level 3: "Sailing close to the wind" or using "loopholes" which were clearly not intended for that purpose. For example, the presentation of bonuses as "loans" which are then written off, so a lower rate of tax applies. Or putting a purchase of a new computer down as a business expense when you'll mostly use it for personal reasons. Morally wrong IMO, although legal.
I would also put in this category facilitating other people's tax evasion i.e. paying in cash in return for a better rate/price.
Level 4: Outright tax evasion, i.e. putting money in offshore accounts. Morally and legally wrong.