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 $249.52   8.817%   $271.5202   $271.52   $271.50   $271.50  
  Purchase #2 $59.60   13.843%   $67.8504   $67.85   $67.85   $67.90  
  Purchase #3 $104.43   10.251%   $115.1351   $115.14   $115.15   $115.10  
  Purchase #4 $21.82   9.629%   $23.921   $23.92   $23.90   $23.90  
  Purchase #5 $91.80   3.542%   $95.0516   $95.05   $95.05   $95.10  
  Purchase #6 $198.78   3.667%   $206.0693   $206.07   $206.05   $206.10  
  Purchase #7 $235.75   8.795%   $256.4842   $256.48   $256.50   $256.50  
  7 purchase totals = $961.70       $1,036.0318   $1,036.03   $1,036.00   $1,036.10