On SourceTree 1.2.8.1 the Spelling and Grammer preference is remembered and honored:
http://screencast.com/t/nhU5uQ7oli
However on SourceTree 1.2.9.1 the if you ask SourceTree to not Correct Speling Automatically SourceTree forgets the preference and commits autocorrects words:
http://screencast.com/t/RllqjbGL