Welcome to the Forum.  Your question is a good one.  I appreciate your acknowledgement that no sexual encounter is 100% risk free as this is the correct starting point. Further, let me point out that partner choice and a commitment to correct and consistent condom use (probably in reverse order) represent major determinants of risk for infection and that, again, as you suggest, choice of sex acts likewise impacts risk .  With that as background:

Sex acts: The "biological efficiency" of transmission of infection varies with sex acts.  In general for a male, if a sex partner is infected, receptive rectal intercourse is the highest risk event, followed by insertive rectal sex, insertive vaginal sex, receipt of oral sex (fellatio) and  performance of oral sex on an infected female partner is likely the lowest risk sort of exposure.  For HIV the differences have been calculated while for most other STIs these are hierarchical but it is difficult to provide a numerical estimate.  The risk for performing oral sex on a female partner is very, very low- for HIV and all other STIs.   I am not aware of anyone acquiring HIV from performing cunnilingus on an HIV infected sex partner.

Trichomonas, gonorrhea and chlamydial infection:  These are the most common treatable STIs.  They can be asymptomatic in men and women.  In men gonorrhea is most often symptomatic for chlamydia and trich we estimate that both can be asymptomatic as much as half the time. 

Syphilis: Is a rare infection and occurs most often among men who have sex with other men (MSM).  Even with commercial sex workers, risk for infection is quite low

Herpes:  Condoms reduce risk for HSV by about 60%  many people have HSV but do not know it.  Risk of herpes for an uninfected partner having genital sex with an infected partner is less than 1 in 1000 and perhaps closer to 1 in 10,000

HIV: Another STI.  Most common in MSM and inefficiently transmitted.  With vaginal sex with an infected partners average risk for infection is between 1 in 1000 and 1 in 2000. 

HPV:  Close to all sexually active persons already have or have had the infection.  Difficult  to counsel regarding avoidance or testing. 

Regarding your question on the need for testing, as long as condoms are routinely used throughout sex, testing after each encounter may not be needed or even advisable.   On the other hand if you are going to regularly engage in sex with partners outside of your marriage, I think it would be a good idea to test every 6-12 months for gonorrhea, chlamydia, syphilis and HIV.  they are fairly "routine" tests and widely available.  Obviously this varies according to your specific situation. 

I hope these comments are helpful to you.  EWH