Let's do some math ..  Rounding examples of 7 random hypothetical purchases:
  Case Tag  
Price  
Sales  
Tax %  
Raw Total  
Incl. Sales Tax  
Rounded  
to Penny  
Rounded  
to Nickel  
Rounded  
to Dime  
  Purchase #1 $205.61   3.718%   $213.2546   $213.25   $213.25   $213.30  
  Purchase #2 $113.77   6.575%   $121.2504   $121.25   $121.25   $121.30  
  Purchase #3 $163.69   7.475%   $175.9258   $175.93   $175.95   $175.90  
  Purchase #4 $223.64   11.777%   $249.9781   $249.98   $250.00   $250.00  
  Purchase #5 $12.51   5.533%   $13.2022   $13.20   $13.20   $13.20  
  Purchase #6 $123.52   13.713%   $140.4583   $140.46   $140.45   $140.50  
  Purchase #7 $81.32   4.852%   $85.2656   $85.27   $85.25   $85.30  
  7 purchase totals = $924.06       $999.3350   $999.34   $999.35   $999.50