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 $170.80   9.401%   $186.8569   $186.86   $186.85   $186.90  
  Purchase #2 $192.98   6.554%   $205.6279   $205.63   $205.65   $205.60  
  Purchase #3 $185.78   8.185%   $200.9861   $200.99   $201.00   $201.00  
  Purchase #4 $180.95   4.209%   $188.5662   $188.57   $188.55   $188.60  
  Purchase #5 $148.52   11.361%   $165.3934   $165.39   $165.40   $165.40  
  Purchase #6 $239.46   6.435%   $254.8693   $254.87   $254.85   $254.90  
  Purchase #7 $167.93   11.132%   $186.624   $186.62   $186.60   $186.60  
  7 purchase totals = $1,286.42       $1,388.9238   $1,388.93   $1,388.90   $1,389.00