Normal procedure when eg. replacing injectors is to loosen the injector fuel line union one by one while cranking, to flush any air out (helps too with flushing
out any tiny debris which might have loosened itself from threaded areas). Have some clean cloth handy to catch/clean the spill.
Air in high pressure fuel lines will create great problems with starting and may prevent starting completely. There should be no bubbles (nor diesel leaks) around properly tightened unions. With tanks
located below engine (normal case) _any_ air leak will cause fuel to return to tank and fill fuel lines with air. Make sure there are no leaks
on either side.
Personally, when I have to disconnect any fuel hose I wet the mating areas with a very thin film of grease before reassembly. Just a wetting, nothing more. I use silicone dielectric grease for this purpose to avoid damaging any rubber. Probably an overkill, as the rubber in question is fuel/oil resistant, but it served me well for years, so why change?