I usually advise not whats technically best, but whats most powerful (especially in PvE), at least until you are approaching 60.
Having said that, however, it doesnt hurt to respec and try out different things. Too many people fixate too much on what the 'best' spec is, but different people have different styles.
If I have another person tell me not to cast shadow spells as a priest, Im going to go postal :twisted: They dont understand that using Vampiric Embrace is like casting Renew on the entire party, and doesnt require me to babysit everybody's health.
I know Rogue also has many different viable specs, which bring different strengths and weaknesses to them. So I guess overall the best advice would be to level up and become familiar with the class, then keep toying with the Talent Calculator
until you become familiar with what the talents are and what they bring to the table.
In my case, I have what I consider to be the 'perfect' priest build, but I admit it's really only perfect for me. Others value different class roles more, and it likely wouldnt be as effective on a PvE server.