[boost] [warnings] Are warnings acceptable artifacts from builds?

Just curious, when I use certain libraries, I get compiler warnings. Are these simply acceptable during a build, or should they be reported to somebody ? The latest warning I'm seeing are: Warning 4 warning C4245: 'initializing' : conversion from 'int' to 'const boost::detail::mask_uint_t<8>::least', signed/unsigned mismatch C:\Trunk.3.1\ThirdParty\boost\crc.hpp 377 Warning 5 warning C4245: 'initializing' : conversion from 'int' to 'const boost::detail::mask_uint_t<16>::least', signed/unsigned mismatch C:\Trunk.3.1\ThirdParty\boost\crc.hpp 400 Regards, -Sid Sacek