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 $234.86   5.809%   $248.503   $248.50   $248.50   $248.50  
  Purchase #2 $17.45   7.369%   $18.7359   $18.74   $18.75   $18.70  
  Purchase #3 $55.41   13.352%   $62.8083   $62.81   $62.80   $62.80  
  Purchase #4 $116.44   7.443%   $125.1066   $125.11   $125.10   $125.10  
  Purchase #5 $106.31   4.731%   $111.3395   $111.34   $111.35   $111.30  
  Purchase #6 $82.62   3.650%   $85.6356   $85.64   $85.65   $85.60  
  Purchase #7 $106.99   8.764%   $116.3666   $116.37   $116.35   $116.40  
  7 purchase totals = $720.08       $768.4955   $768.51   $768.50   $768.40