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 $225.88   6.224%   $239.9388   $239.94   $239.95   $239.90  
  Purchase #2 $114.73   3.259%   $118.4691   $118.47   $118.45   $118.50  
  Purchase #3 $132.49   4.595%   $138.5779   $138.58   $138.60   $138.60  
  Purchase #4 $59.96   4.854%   $62.8705   $62.87   $62.85   $62.90  
  Purchase #5 $99.46   8.106%   $107.5222   $107.52   $107.50   $107.50  
  Purchase #6 $93.68   9.785%   $102.8466   $102.85   $102.85   $102.80  
  Purchase #7 $215.85   13.496%   $244.9811   $244.98   $245.00   $245.00  
  7 purchase totals = $942.05       $1,015.2062   $1,015.21   $1,015.20   $1,015.20