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 $227.45   12.304%   $255.4354   $255.44   $255.45   $255.40  
  Purchase #2 $165.71   13.184%   $187.5572   $187.56   $187.55   $187.60  
  Purchase #3 $113.99   9.322%   $124.6161   $124.62   $124.60   $124.60  
  Purchase #4 $23.82   8.782%   $25.9119   $25.91   $25.90   $25.90  
  Purchase #5 $131.85   3.661%   $136.677   $136.68   $136.70   $136.70  
  Purchase #6 $98.33   6.400%   $104.6231   $104.62   $104.60   $104.60  
  Purchase #7 $184.28   7.419%   $197.9517   $197.95   $197.95   $198.00  
  7 purchase totals = $945.43       $1,032.7724   $1,032.78   $1,032.75   $1,032.80