Option 1: Set the source type of a single raster to "Processed"
In Arc Catalog, open the Raster Dataset Properties of the GeoTiff. Browse to the General tab, and set the Source Type to "Processed".
The different Source Type options are described in the ArcGIS manual:
The Source Type controls how the data is rendered by default. These options will be overridden if you modify the defaults. The options are:
Generic—Uses the application defaults for resampling and stretching.
Elevation—Applies bilinear resampling and a Min-Max stretch.
Thematic—Applies nearest neighbor resampling and a Standard Deviation stretch.
Processed—No stretch is applied.
For more details, have a look at the ArcGIS resource blog article "Stop stretching my imagery please".
Option 2: Edit default stretching for all raster datasets
You can change the default settings of Raster display in the ArcMap Options dialog:
Within the ArcGIS menu, browse to Customize -> ArcMap Options....
In this dialogue, go to the Raster Tab and Raster Layer subtab.
Now check "Enable custom Rendering Defaults" and set Stretch Type and corresponding options as you like. In your case, set Stretch Type to "None" in order to import all your GeoTiff Images without any standard stretching applied.
A more detailed description of opening raster datasets with customized settings can be found in the ArcGIS Resources blog.