This is a good question because one of the nice things about fiction is its ability to dive into the minds of the characters. Including a character’s thoughts‑-so long as you’re not violating the point-of-view boundaries of your story‑-can often reveal complexities, deepen the conflict, or help the reader better understand the situation.
There is no one right way to indicate what a character is thinking. The most important thing is to make it absolutely clear that the narrative has shifted into a thought. Italics are one option, but I think they underline the thought a little too blatantly, like this:
Maria sat at her desk, twirling her pen. Should I make the call now or wait until Monday so I don’t ruin his weekend?
There’s also something inauthentic about thoughts conveyed in complete, direct sentences, since the thought process doesn’t really work that way, and italics imply that’s what’s happening. It’s easy enough, and a bit smoother, to indicate the thought without the italics, as seen in these two examples:
Maria sat at her desk, twirling her pen. Should she make the call now or wait until Monday so as not to ruin his weekend?
Maria sat at her desk, twirling her pen, wondering if she should make the call now or wait until Monday so she didn’t ruin his weekend.
At least, that’s what I think.