Is it worth it to master LaTeX or MS Word?
This depends entirely on the journal policy, which in turn tends to depend strongly on field.  For example:


In computer science and electrical engineering, many ACM and IEEE journals expect papers to be turned in using a format that is very close to publication and then re-format little if at all.  For these journals, good mastery of LaTeX is a must.
Many biology journals, by contrast, will refuse to take nicely formatted submissions and require text to be submitted separately from figures.  For these journals, you mostly just need to know how to use your citation manager.


Beyond looking at individual journals, I would say that a good rule of thumb is to consider how many equations you are likely to publish.  The more mathematical your work, the more likely that your publication venue is to have embraced LaTeX and the more benefit expertise with it is likely to be to you.