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 $244.83   3.548%   $253.5166   $253.52   $253.50   $253.50  
  Purchase #2 $212.84   12.592%   $239.6408   $239.64   $239.65   $239.60  
  Purchase #3 $28.27   8.473%   $30.6653   $30.67   $30.65   $30.70  
  Purchase #4 $155.25   5.240%   $163.3851   $163.39   $163.40   $163.40  
  Purchase #5 $133.26   9.552%   $145.989   $145.99   $146.00   $146.00  
  Purchase #6 $111.65   9.403%   $122.1484   $122.15   $122.15   $122.10  
  Purchase #7 $23.94   7.530%   $25.7427   $25.74   $25.75   $25.70  
  7 purchase totals = $910.04       $981.0879   $981.10   $981.10   $981.00