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 $246.93   12.677%   $278.2333   $278.23   $278.25   $278.20  
  Purchase #2 $116.58   7.307%   $125.0985   $125.10   $125.10   $125.10  
  Purchase #3 $211.38   6.393%   $224.8935   $224.89   $224.90   $224.90  
  Purchase #4 $156.99   5.752%   $166.0201   $166.02   $166.00   $166.00  
  Purchase #5 $233.36   4.311%   $243.4201   $243.42   $243.40   $243.40  
  Purchase #6 $158.29   10.554%   $174.9959   $175.00   $175.00   $175.00  
  Purchase #7 $117.26   5.506%   $123.7163   $123.72   $123.70   $123.70  
  7 purchase totals = $1,240.79       $1,336.3777   $1,336.38   $1,336.35   $1,336.30