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 $88.66   13.152%   $100.3206   $100.32   $100.30   $100.30  
  Purchase #2 $179.98   3.587%   $186.4359   $186.44   $186.45   $186.40  
  Purchase #3 $232.80   10.494%   $257.23   $257.23   $257.25   $257.20  
  Purchase #4 $44.27   9.436%   $48.4473   $48.45   $48.45   $48.40  
  Purchase #5 $205.40   4.549%   $214.7436   $214.74   $214.75   $214.70  
  Purchase #6 $131.68   9.267%   $143.8828   $143.88   $143.90   $143.90  
  Purchase #7 $63.97   10.830%   $70.898   $70.90   $70.90   $70.90  
  7 purchase totals = $946.76       $1,021.9582   $1,021.96   $1,022.00   $1,021.80