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 $8.75   12.804%   $9.8704   $9.87   $9.85   $9.90  
  Purchase #2 $68.70   8.477%   $74.5237   $74.52   $74.50   $74.50  
  Purchase #3 $231.80   6.652%   $247.2193   $247.22   $247.20   $247.20  
  Purchase #4 $214.92   11.530%   $239.7003   $239.70   $239.70   $239.70  
  Purchase #5 $72.43   7.480%   $77.8478   $77.85   $77.85   $77.80  
  Purchase #6 $71.79   8.617%   $77.9761   $77.98   $78.00   $78.00  
  Purchase #7 $113.75   13.394%   $128.9857   $128.99   $129.00   $129.00  
  7 purchase totals = $782.14       $856.1233   $856.13   $856.10   $856.10