Is there a time of day that Tehillim (Psalms) should be recited? Is there a time of day that Tehillim should NOT be recited?
According to the Zohar, Psalms should only be recited during the daytime or after midnight. Thus, according to the Zohar, from nightfall until midnight one should refrain from reciting Psalms (except for Motzei Shabbos). (This ruling applies only to an individual. A community may congregate to recite Psalms together even at night.)
However, most people* are not strict about everything the Zohar says. The Zohar is a work of Kabballah, directed at people already on a high spiritual level to help them reach even greater heights. Therefore, most people are not strict about this (or even know about this) and simply say Psalms whenever they are able (and are definitely accomplishing a big mizvah by doing so).
This idea requires a little more explanation. Although the Zohar is part of Torah tradition, handed down from Sinai, it is not binding on every individual in the same way as the revealed parts of the Torah (like Bible and Talmud). If G-d had wanted the teachings of the Zohar to be followed by every Jew, He would have recorded it as part of the revealed Torah and not as a work of Kabballah.
Of course, if your schedule is flexible and you can plan to say your Psalms during the day, all the better.
Take care,
Shlomo Shulman
Waterbury, CT
*This is true for those of Ashkenazic descent. Those following a different custom, such as those of Sefardic descent, may have a different approach. In either case, consult your local Rabbinic authority for guidance.