In general it is not bad practice to offset the waveform to measure a small part of it. As other posters have mentioned it can affect the accuracy of measurement.
Good scopes can often do 8-10 screen heights of offset but I've used some that suffer problems with only 3-4 screens of offset.
There is usually a specification for the amount of allowed offset. In the case of the Rigol DS1054Z it is described as Offset Range:
This is the allowable offset that will still meet the specifications described elsewhere in the specification.
Usually the sort of effects I see are the baseline wander but you may also see changes in dynamic response.
Make sure that the probe is compensated correctly if you are offsetting a significant amount or you may get an error in voltage measurements - I've been bitten by that a couple of times wondering why voltages are not what I expect.
Rigol DS1054Z specification