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 $20.60   7.288%   $22.1013   $22.10   $22.10   $22.10  
  Purchase #2 $39.56   9.859%   $43.4602   $43.46   $43.45   $43.50  
  Purchase #3 $81.34   7.197%   $87.194   $87.19   $87.20   $87.20  
  Purchase #4 $93.47   11.550%   $104.2658   $104.27   $104.25   $104.30  
  Purchase #5 $145.36   9.532%   $159.2157   $159.22   $159.20   $159.20  
  Purchase #6 $57.60   3.785%   $59.7802   $59.78   $59.80   $59.80  
  Purchase #7 $102.78   5.849%   $108.7916   $108.79   $108.80   $108.80  
  7 purchase totals = $540.71       $584.8088   $584.81   $584.80   $584.90