Use masking tape to make a neat line when you recaulk the shower or tub. - How to Re-caulk a Shower or Bathtub: http://www.familyhandyman.com/bathroom/remodeling/how-to-re-caulk-a-shower-or-bathtub/view-all
Scraping out moldy caulk from around a bathtub is a tough, tedious job. And if you get too aggressive, you'll chip a tile or mar the tub finish. Make the job a lot easier by first softening the caulk with a caulk remover.