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 $17.70   3.108%   $18.2501   $18.25   $18.25   $18.30  
  Purchase #2 $183.28   11.386%   $204.1483   $204.15   $204.15   $204.10  
  Purchase #3 $79.82   9.429%   $87.3462   $87.35   $87.35   $87.30  
  Purchase #4 $111.86   5.461%   $117.9687   $117.97   $117.95   $118.00  
  Purchase #5 $107.60   13.735%   $122.3789   $122.38   $122.40   $122.40  
  Purchase #6 $196.80   13.230%   $222.8366   $222.84   $222.85   $222.80  
  Purchase #7 $204.75   6.204%   $217.4527   $217.45   $217.45   $217.50  
  7 purchase totals = $901.81       $990.3815   $990.39   $990.40   $990.40