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 $207.59   12.839%   $234.2425   $234.24   $234.25   $234.20  
  Purchase #2 $97.25   5.834%   $102.9236   $102.92   $102.90   $102.90  
  Purchase #3 $59.36   13.593%   $67.4288   $67.43   $67.45   $67.40  
  Purchase #4 $189.39   5.464%   $199.7383   $199.74   $199.75   $199.70  
  Purchase #5 $250.44   12.637%   $282.0881   $282.09   $282.10   $282.10  
  Purchase #6 $25.90   13.192%   $29.3167   $29.32   $29.30   $29.30  
  Purchase #7 $28.85   9.544%   $31.6034   $31.60   $31.60   $31.60  
  7 purchase totals = $858.78       $947.3414   $947.34   $947.35   $947.20