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 $249.93   13.612%   $283.9505   $283.95   $283.95   $284.00  
  Purchase #2 $136.33   12.169%   $152.92   $152.92   $152.90   $152.90  
  Purchase #3 $219.26   13.694%   $249.2855   $249.29   $249.30   $249.30  
  Purchase #4 $97.85   5.155%   $102.8942   $102.89   $102.90   $102.90  
  Purchase #5 $122.54   11.307%   $136.3956   $136.40   $136.40   $136.40  
  Purchase #6 $246.34   9.219%   $269.0501   $269.05   $269.05   $269.10  
  Purchase #7 $149.32   5.266%   $157.1832   $157.18   $157.20   $157.20  
  7 purchase totals = $1,221.57       $1,351.6791   $1,351.68   $1,351.70   $1,351.80