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 $13.74   12.236%   $15.4212   $15.42   $15.40   $15.40  
  Purchase #2 $76.96   4.678%   $80.5602   $80.56   $80.55   $80.60  
  Purchase #3 $14.74   6.530%   $15.7025   $15.70   $15.70   $15.70  
  Purchase #4 $81.27   10.234%   $89.5872   $89.59   $89.60   $89.60  
  Purchase #5 $78.73   7.611%   $84.7221   $84.72   $84.70   $84.70  
  Purchase #6 $164.66   7.706%   $177.3487   $177.35   $177.35   $177.30  
  Purchase #7 $75.46   11.510%   $84.1454   $84.15   $84.15   $84.10  
  7 purchase totals = $505.56       $547.4873   $547.49   $547.45   $547.40