When a new Content Type SPI is added in a SharePoint project, the content type ID generated by Visual Studio has GUID in lower case ie, 0x010100F67935030963E344B2D6443E349B383100E85796468D2B3A4783E2B15F77D13D6A090015A1B1F5C494412C911374A2AEF4988C0076cafce4d1e94306ab803af479bb440b
Notice that the entire content type ID is in uppper case except the last part "76cafce4d1e94306ab803af479bb440b" is in lower case. This last part is generated by Visual Studio.
One of the issues of having ID in lower case (as faced by me) is when you create custom actions based (e.g. custom ribbon buttons, tabs) on content type ID.
Upon deployment, SharePoint internally converts all the content type id's in upper case and stores in content DB. For any content type based custom action, the ID's mentioned by the developer in code will then be compared with the ID's stored in SharePoint.
This comparison (lower to upper) will always result in a failure and you wont be able to see your custom action assocaited/visible.