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 $227.69   13.838%   $259.1977   $259.20   $259.20   $259.20  
  Purchase #2 $117.54   5.866%   $124.4349   $124.43   $124.45   $124.40  
  Purchase #3 $80.69   6.141%   $85.6452   $85.65   $85.65   $85.60  
  Purchase #4 $201.56   3.521%   $208.6569   $208.66   $208.65   $208.70  
  Purchase #5 $24.80   3.396%   $25.6422   $25.64   $25.65   $25.60  
  Purchase #6 $106.62   3.459%   $110.308   $110.31   $110.30   $110.30  
  Purchase #7 $9.95   6.256%   $10.5725   $10.57   $10.55   $10.60  
  7 purchase totals = $768.85       $824.4574   $824.46   $824.45   $824.40