Situational matters can also reduce the PRE dice rolled - that's the flip side of the bonus dice you mention. Reputation might reduce the dice rolled if the target doesn't believe what the character is saying is in character ("Sorry, Batman, I know you'd never pull that trigger!"). Striking appearance will only help if appropriate to the situation... if Captain Monster (Striking Appearance - Hideous Appearance, Reputation - Unholy Beast) is trying to calm down a panicking crowd (PRE attack) they should get a penalty, while Golden Angel (Striking Appearance - Unearthly Beauty, Reputation - Paragon of Good) will probably get a bonus. If the PRE attack was trying to make them flee in terror, the modifiers would be different.
Probably also worth pointing out that the lower tier PRE effects may have no practical effect, especially the first tier (act before the target this phase, +5 PRE to resist contrary PRE attacks). A character that loses half a phase due to PRE+10 can still attack, and "considering very deeply what the character says" will often mean little ("Throw down your guns, villains!" "Aargh! It's the Black Prowler! GET AWAY!" *bang* *bang* *bang*).
PRE also defends better than it attacks - each 5 points of PRE gives 1d6 to attack, but 5 points to defense. Without modifiers, same-PRE characters are unlikely to impress each other much.
Unless a character is deliberately built to be timid, PRE 15 is probably a minimum for PCs in either Heroic or Superheroic. 20 would be fine for a character whose description makes a point of their leadership, charisma or domination. PRE 15 needs about 7d6 to get to +10, around 10d6 to break +20 (where it really becomes a Power). To reliably get +20 against PRE 20 you would need 12d6 or more, which is going to either require PRE as a major power, or some very favourable circumstances.