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 $39.28   8.440%   $42.5952   $42.60   $42.60   $42.60  
  Purchase #2 $26.56   8.737%   $28.8805   $28.88   $28.90   $28.90  
  Purchase #3 $172.48   9.212%   $188.3689   $188.37   $188.35   $188.40  
  Purchase #4 $146.50   10.165%   $161.3917   $161.39   $161.40   $161.40  
  Purchase #5 $60.88   11.281%   $67.7479   $67.75   $67.75   $67.70  
  Purchase #6 $108.94   12.192%   $122.222   $122.22   $122.20   $122.20  
  Purchase #7 $16.49   9.424%   $18.044   $18.04   $18.05   $18.00  
  7 purchase totals = $571.13       $629.2502   $629.25   $629.25   $629.20