That sort of depends what you want to do at whatever place you want to go to beyond Earth orbit. If budgets were not a constraining factor, you would simultaneously build the biggest rocket technology would allow, megawatt class in-space propulsions systems for cargo pre-deployment, nuclear thermal in-space stages for crew transport, long duration in-space habitats and capable destination systems to make the people as productive and self- sufficient as possible for any mission at the chosen destination.
If you are talking about going to Mars, a heavy-lift rocket simplifies the mission architecture by allowing more systems to be integrated into mission vehicles that enable contingencies to be mitigated without having to rely on another launch or rendezvous. Even with a heavy-lift launch vehicle, there are too many launches (six to twelve) required to pull off a conjunction class mission with about a 500 day surface stay if everything is thrown away after each mission. Decreasing the size of the launch vehicle just multiplies the problem. However, I know of no place on Earth where budgets are not a factor!
The development of a new launch vehicle (especially a heavy-lift rocket) is one of the most expensive aspects of space exploration, and the operational costs of heavy lift facilities and vehicles are covered just by the missions that use them, leaving a small portion of the total budget to develop everything else. If the mission that truly required a heavy-lift vehicle were 30 years away, it would make sense to leverage existing launch vehicles to extend our reach into cislunar space, build up our long duration experience on the ISS from months to years and take the funding that would have been spent on heavy-lift and invest that in all the other capabilities humans will need to thrive in space. Then ten years before humans proceed to that new challenging destination, begin development of a heavy-lift vehicle that could leverage all the other technology developments before it.
Answered by Patrick Troutman, NASA futurology expert