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 $78.81   8.223%   $85.2905   $85.29   $85.30   $85.30  
  Purchase #2 $217.52   11.255%   $242.0019   $242.00   $242.00   $242.00  
  Purchase #3 $155.96   5.571%   $164.6485   $164.65   $164.65   $164.60  
  Purchase #4 $233.94   3.871%   $242.9958   $243.00   $243.00   $243.00  
  Purchase #5 $27.87   3.138%   $28.7446   $28.74   $28.75   $28.70  
  Purchase #6 $193.87   8.330%   $210.0194   $210.02   $210.00   $210.00  
  Purchase #7 $94.75   7.425%   $101.7852   $101.79   $101.80   $101.80  
  7 purchase totals = $1,002.72       $1,075.4859   $1,075.49   $1,075.50   $1,075.40