When we create a pipeline, it'd help if we could check the same documents data in the different stages ( now their preview is from randomly selected documents AFAIK ), so it's inconvenient to inspect one specific documents preview phases through all the stages.
Possible solutions:
1) lock the first stage's documents' order and apply it to the following stages (if possible, eg. if the stage is $group, then we can not apply the rule to the next stage)
2) set a sorting ( if possible ) of documents. But only for the preview.