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 $138.28   13.599%   $157.0847   $157.08   $157.10   $157.10  
  Purchase #2 $141.75   4.558%   $148.211   $148.21   $148.20   $148.20  
  Purchase #3 $71.30   8.195%   $77.143   $77.14   $77.15   $77.10  
  Purchase #4 $19.35   3.349%   $19.998   $20.00   $20.00   $20.00  
  Purchase #5 $12.25   10.146%   $13.4929   $13.49   $13.50   $13.50  
  Purchase #6 $108.53   5.554%   $114.5578   $114.56   $114.55   $114.60  
  Purchase #7 $99.31   5.318%   $104.5913   $104.59   $104.60   $104.60  
  7 purchase totals = $590.77       $635.0787   $635.07   $635.10   $635.10