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 $108.84   12.621%   $122.5767   $122.58   $122.60   $122.60  
  Purchase #2 $119.59   11.695%   $133.5761   $133.58   $133.60   $133.60  
  Purchase #3 $163.53   6.874%   $174.7711   $174.77   $174.75   $174.80  
  Purchase #4 $77.47   13.648%   $88.0431   $88.04   $88.05   $88.00  
  Purchase #5 $59.50   8.554%   $64.5896   $64.59   $64.60   $64.60  
  Purchase #6 $56.99   11.503%   $63.5456   $63.55   $63.55   $63.50  
  Purchase #7 $34.55   3.727%   $35.8377   $35.84   $35.85   $35.80  
  7 purchase totals = $620.47       $682.9399   $682.95   $683.00   $682.90