Hardie recommends a 1/8" gap from siding to trim, but no gap between butt joints. I worked for a company though that we took a 6d hand drive sider and gapped between butt joints though. As to caulking Hardie does not recommend the use of caulk on field joints. Because of this I use joint flashing behind the field joints, pin the corners of the abutting pieces (no shear, just aesthetics, it keeps the siding corners down and lined up) and caulk it with a 50 year elastomeric caulk.