Titles, rather like Still Images in the Timeline, are "synthetic Video," so for the ultimate playback, one would Render them. Now, I seldom find playback of Un-Rendered Titles to be an issue, and usually on do so, if I have adjusted, say Effect>Motion>Position, or Motion>Scale, or similar. Then, I might Render that portion of the Timeline, and maybe several times, if I am adjusting the Effects' parameters.
Do you "have to" render them because they just don't play smooth, or do you "have to" because the indicator above the title turns red?
It sounds like a settings problem because I can add multiple titles without the yellow bar turning red. I tried creating titles and then putting them in a timeline using a different frame size. Still not a problem.
I suggest that you create a new project, import one of the files from the original project, add a few titles and see what happens.
It may be that one project at fault.