Should beta blockers be held or reduced when patients are hospitalized with acute decompensated heart failure?

No, according to this study:

Conclusion: In conclusion, during ADHF, continuation of beta-blocker therapy is not associated with delayed or lesser improvement, but with a higher rate of chronic prescription of beta-blocker therapy after 3 months, the benefit of which is well established.

In the vast majority of subjects the beta blocker dose was not changed. Patients requiring drugs like dobutamine were excluded.

According to this Medscape commentary the OPTIMIZE-HF registry also supports beta blocker continuation:

B-CONVINCED is consistent with an analysis from the OPTIMIZE-HF registry [3], in which maintenance of beta-blocker therapy in systolic ADHF dramatically cut mortality at two to three months (p=0.044), as did starting the drugs at discharge in such patients who hadn't previously been on them (p=0.006). Also as reported by heartwire when the study was published, taking patients off beta blockers at hospitalization more than doubled all-cause mortality (p=0.013).