The debate over FE vs. RE often reflects around the lack of understanding of the concepts behind the models. To make matters worse, the Cochrane handbook only briefly touches on this (9.4.2 Principles of meta-analysis) and provides some guidance on whether or not the posthoc decision to use the FE or the RE was the correct one (10.4.4.1 Comparing fixed and random-effects estimates). From the Handbook: "The combination of intervention effect estimates across studies may optionally incorporate an assumption that the studies are not all estimating the same intervention effect, but estimate intervention effects that follow a distribution across studies. This is the basis of a random-effects meta-analysis (see Section 9.5.4). Alternatively, if it is assumed that each study is estimating exactly the same quantity a fixed-effect meta-analysis is performed."
In only the rarest types of meta-analyses (e.g. same trial repeated multiple times with the same exact protocol) could we assume that the studies are all estimating the same intervention effect, with between-study differences in results attributed to random error. On the other hand, we typically combine studies that use similar populations, similar interventions, similar comparisons, similar doses, etc. assuming that the studies have a reasonable amount of clinical homogeneity allowing their data to be pooled together.
Therefore, IMHO, the decision to use the FE or RE model should be performed at the protocol stage since the actual results do not play a factor in the model choice. Having said that, as noted in section 10.4.4.1 of the Handbook, this choice may need to be reversed after seeing the results of the meta-analysis; if there is suspicion of small-study effect skewing the pooled results.
So, in closing, unless there is justification for the use of FE model, the RE model should be the default choice when conducting a meta-analysis from different study results. Of course, that doesn't mean you can't test test the robustness of the analyses in a sensitivity analysis (FE model). Actually a lot of Cochrane groups request this.