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 $19.31   8.703%   $20.9905   $20.99   $21.00   $21.00  
  Purchase #2 $88.95   9.810%   $97.676   $97.68   $97.70   $97.70  
  Purchase #3 $40.98   10.306%   $45.2034   $45.20   $45.20   $45.20  
  Purchase #4 $45.54   5.834%   $48.1968   $48.20   $48.20   $48.20  
  Purchase #5 $44.47   11.703%   $49.6743   $49.67   $49.65   $49.70  
  Purchase #6 $201.61   10.115%   $222.0029   $222.00   $222.00   $222.00  
  Purchase #7 $65.80   5.873%   $69.6644   $69.66   $69.65   $69.70  
  7 purchase totals = $506.66       $553.4083   $553.40   $553.40   $553.50