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 $131.94   5.340%   $138.9856   $138.99   $139.00   $139.00  
  Purchase #2 $105.95   3.333%   $109.4813   $109.48   $109.50   $109.50  
  Purchase #3 $55.86   10.405%   $61.6722   $61.67   $61.65   $61.70  
  Purchase #4 $22.54   4.387%   $23.5288   $23.53   $23.55   $23.50  
  Purchase #5 $45.31   9.121%   $49.4427   $49.44   $49.45   $49.40  
  Purchase #6 $45.73   10.520%   $50.5408   $50.54   $50.55   $50.50  
  Purchase #7 $67.82   9.392%   $74.1897   $74.19   $74.20   $74.20  
  7 purchase totals = $475.15       $507.8411   $507.84   $507.90   $507.80