What's the pros and cons of saying I want a better salary?
The Pros are : If your talents and experiences truly deserve a better salary, the potential employer may give you a higher salary.
However, the potential employer may attempt to negotiate with you if their budget is limited. So, it may take some time to negotiate.
The Cons are: If your talents and experiences don't really deserve a better salary, then the potential employer may reject your application right away, and choose another candidate. If they already pick another candidate, then you can't ask them to go back and hire you with their original offer.
However, if the employer have a difficult time finding another candidate, they may attempt to negotiate with you. So, it may take some time to negotiate.
I can't really say it's because the current job sucks.
It's true. You should not say negative things about your current job, boss, coworkers, or company. Never say negative things like those in an interview because the interviewers may not have good impressions about you.
How do I know when/if it's okay to say it's because I want to earn a better salary??
The best way to earn a better salary is to provide good justifications to potential employers.
First, you need to do serious research on the web to find out what a person with similar skills and experiences as yours earn in the city that the company is located (if this is an onsite and not a remote position).
A fair salary depends on the skills, experiences of the candidates, and location of the companies as the cost of livings varies from one city to another.
There are many websites such as Glass Door, etc... that provides this salary info.
Then, when you go to an interview, you should be able to provide a professional, precise, and factual argument as to why you deserve a better salary based on your skills and experience (in a specific market/city). Again, you should be able to back up your argument with your research on the web.