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 $238.29   12.448%   $267.9523   $267.95   $267.95   $268.00  
  Purchase #2 $133.92   12.476%   $150.6279   $150.63   $150.65   $150.60  
  Purchase #3 $28.97   9.756%   $31.7963   $31.80   $31.80   $31.80  
  Purchase #4 $183.93   12.357%   $206.6582   $206.66   $206.65   $206.70  
  Purchase #5 $21.41   11.645%   $23.9032   $23.90   $23.90   $23.90  
  Purchase #6 $167.25   11.184%   $185.9552   $185.96   $185.95   $186.00  
  Purchase #7 $50.37   3.687%   $52.2271   $52.23   $52.25   $52.20  
  7 purchase totals = $824.14       $919.1202   $919.13   $919.15   $919.20