Answer from Russell, SPHR, SHRM-SCP:
Discussing wages or salary is considered protected activity under the National Labor Relations Act (NLRA), so you should not take any action – in policy or in practice – to prohibit employees from discussing their pay, nor should you discipline employees for doing so.
More specifically, Section 7 of the NLRA protects the rights of employees to act together to try to improve their pay and working conditions or to fix job-related problems. Although it may not seem like they are trying to improve things by complaining, their discussions are very much a protected right. The National Labor Relations Board (which enforced the NLRA) has been ruling in favor of employees on this this matter since the 80s.
I therefore recommend that you remove the language from your handbook about wage information being confidential. Your best defense against tension and complaining about wages is to ensure that pay rates are fair and that any differences in wages between employees in similar jobs have legitimate justification.