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 $92.83   12.113%   $104.0745   $104.07   $104.05   $104.10  
  Purchase #2 $235.72   9.312%   $257.6702   $257.67   $257.65   $257.70  
  Purchase #3 $65.80   3.805%   $68.3037   $68.30   $68.30   $68.30  
  Purchase #4 $102.43   12.716%   $115.455   $115.46   $115.45   $115.50  
  Purchase #5 $155.96   12.779%   $175.8901   $175.89   $175.90   $175.90  
  Purchase #6 $23.29   5.152%   $24.4899   $24.49   $24.50   $24.50  
  Purchase #7 $189.85   11.243%   $211.1948   $211.19   $211.20   $211.20  
  7 purchase totals = $865.88       $957.0782   $957.07   $957.05   $957.20