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 $18.92   5.610%   $19.9814   $19.98   $20.00   $20.00  
  Purchase #2 $170.87   7.438%   $183.5793   $183.58   $183.60   $183.60  
  Purchase #3 $5.91   10.252%   $6.5159   $6.52   $6.50   $6.50  
  Purchase #4 $208.47   8.182%   $225.527   $225.53   $225.55   $225.50  
  Purchase #5 $123.29   5.460%   $130.0216   $130.02   $130.00   $130.00  
  Purchase #6 $198.37   11.384%   $220.9524   $220.95   $220.95   $221.00  
  Purchase #7 $11.44   6.170%   $12.1458   $12.15   $12.15   $12.10  
  7 purchase totals = $737.27       $798.7234   $798.73   $798.75   $798.70