If you are not involved in an intimate relationship with an uninfected partner, then it is not necessary to take Valtrex or Acyclovir. Howevever, you have a moral obligation (and legal action can be taken against you if you don't) to inform all partners before starting a sexual relationship that you are infected with genital herpes.
According to an article in the New York Times, http://consults.blogs.nytimes.com/2010/06/18/giving-your-partner-herpes/, a condom alone is roughly 30% effective. Antiviral drugs alone are roughly 49% effective. Even if you do both, you still must inform your partner and make it clear the risk of getting herpes is always there.
The person who infected me was asymptomatic and he denied having any STDs after we spent several months getting to know one another. He was a medical doctor. Within two years, I was infected with herpes. Obviously, the relationship ended from lack of trust. Five years later on Acyclovir, I have monthly outbreaks that last 10 days due to a weakened immune system.
Thanks for the comments.I'm not dating anyone at the moment
You are so lucky. The only reason for taking valtrex or acyclovir it to have relief from the frequency of outbreaks (many of us have frequent and painful outbreaks) or to reduce the frequency of asymptomatic shedding so that an unifected partner has a better chance of not getting infected with the herpes virus. I am not involved with anyone but because I have a weakened immune system, I take acyclovir to reduce the frequency and the duration of outbreaks. If I didn't have outbreaks, I wouldn't take it.