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 $205.43   11.482%   $229.0175   $229.02   $229.00   $229.00  
  Purchase #2 $19.97   3.293%   $20.6276   $20.63   $20.65   $20.60  
  Purchase #3 $108.40   9.582%   $118.7869   $118.79   $118.80   $118.80  
  Purchase #4 $78.39   5.819%   $82.9515   $82.95   $82.95   $83.00  
  Purchase #5 $173.61   13.528%   $197.096   $197.10   $197.10   $197.10  
  Purchase #6 $82.46   9.618%   $90.391   $90.39   $90.40   $90.40  
  Purchase #7 $102.62   10.841%   $113.745   $113.75   $113.75   $113.70  
  7 purchase totals = $770.88       $852.6155   $852.63   $852.65   $852.60