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 $20.80   6.706%   $22.1948   $22.19   $22.20   $22.20  
  Purchase #2 $18.46   7.114%   $19.7732   $19.77   $19.75   $19.80  
  Purchase #3 $51.53   13.777%   $58.6293   $58.63   $58.65   $58.60  
  Purchase #4 $170.39   12.552%   $191.7774   $191.78   $191.80   $191.80  
  Purchase #5 $28.73   4.718%   $30.0855   $30.09   $30.10   $30.10  
  Purchase #6 $119.80   13.426%   $135.8843   $135.88   $135.90   $135.90  
  Purchase #7 $29.84   10.752%   $33.0484   $33.05   $33.05   $33.00  
  7 purchase totals = $439.55       $491.3929   $491.39   $491.45   $491.40