This ultimately depends on your own computing habits. If you constantly download free screensavers and mouse pointers, you'll probably find a WINE-compatible virus in no time. However, if you stick to malware-free programs, you're pretty much safe. The best way to ensure that no malware is installed is by exclusively using Free Software or Open Source Software, as the temptation to add malware in such programs is much less than Proprietary Software, as there is less chance of being able to do so successfully.
Please note running a virus on WINE is much safer than running that same virus on Windows, since WINE prevents Windows programs from having Super User privileges, so critical Ubuntu files cannot be (easily) modified. If you want to remove a virus, you can erase the entire WINE prefix (which is the equivalent of wiping an entire Windows installation, except it can be removed and reinstalled from scratch much quicker), wiping the virus with it.