This article is from the comp.sys.ibm.pc.hardware.video Frequently Asked Questions, by Michael Scott with numerous contributions by others. (v1.0).
Since components do change value when they warm up, some minor change in position and size may be expected. How much drift occurs really depends on many factors including the basic design, quality of components, ventilation/cooling, etc. Of course, it is possible to have a monitor that has a component that is worse with respect to temperature. Could also be related to line voltage depending on the regulation of your monitor's power supplies.
In general, my feeling is that if it is not objectionable (a 1/2" shift would be objectionable) AND it's severity is not changing with time, you can ignore it.
Many monitors do this. TVs do this but you are not aware of it since they are already 5-10% overscanned for just this reason, as well as compensating for component aging and line voltage fluctuations.
 
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