It's better form to hold your gut inward (suck-in your abs) while doing ab exercises -- this better form also benefits the lower back. Don't hold your breath, just gently suck in the gut & do the crunch or sit-up.
Note: if doing sit-ups, then have your butt on a fairly well-padded surface (use folded towels if necessary), else you'll harm your low-back in the long-run. The famous boxer, Jack Dempsey wrote of doing sit-ups on a soft surface in his book "CHAMPIONSHIP FIGHTING: Explosive Punching and Aggresive Defence" © 1950/78/83 ISBN 0-913111-00-7 -- not knowing this I did a lot of sit-ups (often daily) on concrete, asphalt, & hard-packed dirt during my 11-years with the US Marines & I have a bad low-back to show for it. The Marines eventually switched to crunches since a soft surface isn't necessary to do them safely.
Always go for good form, else you invite injury (either immediate, or down the road, on this I speak from much bad experience)... you'll also get much better results than using poor form (my best results come after I learn an exercise's proper form -- & many are the instructors that don't know proper form, so be wary... & note that I'm still learning myself). If something feels a little off, then chances are you shouldn't go there any more -- change so that all aspects of a repetition feel good... & take it slow & easy with new exercises (don't rush into using high-reps or heavy weights). Well, that's my opinion.
Take Care,
Marooned Mike