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 $250.27   5.573%   $264.2175   $264.22   $264.20   $264.20  
  Purchase #2 $163.49   11.844%   $182.8538   $182.85   $182.85   $182.90  
  Purchase #3 $10.57   12.652%   $11.9073   $11.91   $11.90   $11.90  
  Purchase #4 $22.85   13.214%   $25.8694   $25.87   $25.85   $25.90  
  Purchase #5 $65.31   8.639%   $70.9521   $70.95   $70.95   $71.00  
  Purchase #6 $20.61   5.871%   $21.82   $21.82   $21.80   $21.80  
  Purchase #7 $239.95   8.325%   $259.9258   $259.93   $259.95   $259.90  
  7 purchase totals = $773.05       $837.5459   $837.55   $837.50   $837.60