No, you don't have to practice reading tategaki.
In Japanese, the term tategaki (
縦書き) is used to describe text written vertically.
This style of writing is read from top to bottom, then right to left,
and was traditionally the standard for Japanese writing.
However, in modern times,
horizontal writing is more common due to the prevalence of the internet and other digital media.
Modern Japanese people often read both horizontal and vertical text.
You don't have to go out of your way and practice reading text written vertically. It is not something to be overly concerned about. As someone learning Japanese, it is unavoidable to come across vertical text, just like it's hard to avoid reading text with kanji. So as long as your immersion is balanced, and you regularly read manga or light novels, you're set. In light novels and manga text is always vertical, maybe with some exceptions.
If you only read on a computer screen, it is possible that the program you are using to read books shows the text horizontally and does not support tategaki. If so, don't worry about it too much, as manga alone will still provide you with sufficient practice to become proficient in reading tategaki.
Reading tategaki is not difficult. I find that reading both vertical and horizontal Japanese is natural, and there is no difference in terms of difficulty. Even if you totally avoid media that has tategaki, it is unlikely to have a major negative impact on your reading ability. You still will be able to read vertical Japanese text, much like you can read vertical English text. Maybe your reading speed will be a bit slower, but it's not something that trips people off.
Ultimately, there are more important and challenging aspects of Japanese that could benefit from focus and deliberate practice.