Depends in which stage you are, but if you are still figuring out what to build, or how to build it, fast-built messy monoliths are the least of your problems.
Once things are validated, opportunities compound to rewrite "properly" (^Wtemporarily) things around or aside the monolith.
Exactly, in any moderately sized corporation new functionality will be rewritten several times, sometimes deprecated/reshuffled completely and crystalized after some time – that is good moment to start thinking about extracting things. Changes on the service boundaries are much more expensive than internal changes.
Once things are validated, opportunities compound to rewrite "properly" (^Wtemporarily) things around or aside the monolith.