There are a few minor issues running any version Linux in VirtualBox. Depending on your hardware and host OS you might not be able to get 3D graphic acceleration to work. For example, some KDE desktop effects may not work, or may be very slow. VirtualBox may have problems with "gpm" and mouse cursor positioning when trying to copy and paste text on a Linux virtual console terminal. Make sure that you have the correct kernel headers installed before you build the VirtualBox guest additions. Slackware should install the correct kernel headers, so you only need to be careful if you build your own kernel and don't build the guest additions against the kernel you are booting.
For Slackware, I use the generic Linux guest additions for VirtualBox rather than the ones for a specific Linux distro.