It depends on whether you have a large enough sample as offering a “DK/NO” will most likely reduce your effective sample size. After all, a part of your respondents will opt for the “DK/NO” and, as a consequence, will most likely not be included in the analysis. The reduced impact in the data will be even stronger for certain socio-demographic groups. It has for instance been shown that lower educated respondents tend to opt for the “DK/NO” far more often than higher educated respondents.