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 $39.95   6.114%   $42.3925   $42.39   $42.40   $42.40  
  Purchase #2 $147.34   10.192%   $162.3569   $162.36   $162.35   $162.40  
  Purchase #3 $104.37   8.647%   $113.3949   $113.39   $113.40   $113.40  
  Purchase #4 $190.37   11.473%   $212.2112   $212.21   $212.20   $212.20  
  Purchase #5 $48.80   11.373%   $54.35   $54.35   $54.35   $54.40  
  Purchase #6 $220.78   11.747%   $246.715   $246.72   $246.70   $246.70  
  Purchase #7 $193.86   4.129%   $201.8645   $201.86   $201.85   $201.90  
  7 purchase totals = $945.47       $1,033.2850   $1,033.28   $1,033.25   $1,033.40