Making changes to your investment portfolio simply for tax purposes is never a cut-and-dried decision, and I've never been one to suggest you should let the tax tail wag the portfolio dog.

That being said, it would be a shame to end a year that will give us portfolio losses—barring some miracle—and also end up paying taxes on funds' year-end distributions without trying to at least minimize the impact.

Vanguard recently said that it doesn't expect to make big, or many, capital gains distributions this year, but you never know, and it's always good to be prepared for the unknown. To that end, here are a few things to think about if you think you want to realize some losses in your portfolios now. (See also: "5 Vanguard Funds to Sell Now.")

First, of course, don't go taking losses in a fund that you'll have trouble buying back into. Remember that, because of Vanguard's limits on trading and investments into closed funds, selling may not be to your advantage if you can't put the bulk of your money back into the fund.

For instance, non-Flagship shareholders are limited in their ability to add to funds such as Capital Opportunity (VHCOX) and PRIMECAP (VPMCX). You wouldn't want to sell off $50,000 worth of Capital Opportunity shares and then be limited to adding back just half that amount.

Second, don't fall prey to the washsale rules. In short, if you sell shares at a loss within a 30-day window on either side of a purchase of shares in that same fund (whether through reinvestment or outright purchase—it doesn't matter), you will not be allowed to realize that loss for tax purposes. The wash-sale rule's intent is to prevent you from selling a fund or stock, say, for a loss on Monday and then repurchasing it on Tuesday.

The wash-sale period is actually a 61-day window which includes the day you sold your fund (or stock or bond) at a loss plus the 30 calendar days on either side of it. The rule concerning a purchase within 30 days prior to the sale has to do with "replacement shares." If you have purchased identical "replacement shares" within the 30-day window prior to the sale, this would disallow taking the loss.

For instance, if you own 100 shares of Fund XYZ at a cost of $50 and you buy another 100 shares on November 1 at $40, then sell 100 shares on November 3 at $41. You try to take the loss on the $50 shares but you can't because you bought 100 identical shares just two days before.

That's a no-no…