In the start of your career if you're young, most would lowball as a last resort, if you have a medium to large body of work in your portfolio, probably be honest.

Make sure to account / factor in for equipment, taxes, family, rent, etc in your cost estimates, as they say estimation in software is hard.

If you lowball, IMHO you're not being honest with yourself and you may not be motivated to do the job. It is also hard to negotiate higher if you keep lowballing estimates with your clients in the long run.