I have had WG tell me they have been tested, usually its when I say to them "We will use condoms for everything including oral? I just like to be on the safe side". They normally then agree but tell me they have been regularly tested.
The ones to watch out for are HPV and Herpies. If you use a condom correctly and it stays intact then there is almost no risk of STI's that are spread by fluids. But you can never say 0 risk.
HPV which causes genital warts can be passed on by skin to skin contact. You can become vaccinated against 90+% of HPV strains that cause warts. If you do that and wear a condom, its very unlikely you will get them. It should be noted that most people have the HPV virus already as its common and experts can't say for sure that you can't get it from other skin to skin contact. For instance if you do MMA, wrestling or get massages. Most older people have had it and their body has gained some immunity from it over time. This is why most over 35s are not advised to get the vaccine. I expect most SP's have got HPV and have probably got some immunity to it unless they are very young and new to the game.
Herpies can be passed on from skin to skin contact too. Its most infectious when you can see the lesions, so if you can't see a rash then you're much safer. If you wear a condom it will protect the parts of your penis where the skin is thinnest and the most susceptible to infection. People with Herpies can invisibly shed the disease, however this is only a small amount of the time and when they do this it is less infectious and its much less likely to penetrate the thicker skin around the areas not covered by a condom. So again if you wear a rain coat and make sure the SP does not have a rash your chances of getting it are very low. (Again never 0).
There are other things you can get but they are much rarer.
I have a friend who is a doctor that works in an STI clinic, this info came from him. He does not know I punt and did think it was a little strange that I was so interested in his occupation
I hope this helps some peoples piece of mind. Remember there is a risk even if its a very small one.