I only bold my speeches when my characters are either...

1, Is transformed and in a suit of armor to simulate how their voice would sound behind it (you see this all the time in Iron Man and, say, Super Sentai/Kamen Rider)
2, If the character is shouting
3, If there's some possibly magic going on that is empowering the person

I never bold it completely. I also like to use colors to best match a certain power set. For example, a character in an orange suit of armor will have orange text or a person empowered with a purple aura will have purple text.