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 $32.62   12.241%   $36.613   $36.61   $36.60   $36.60  
  Purchase #2 $73.34   11.272%   $81.6069   $81.61   $81.60   $81.60  
  Purchase #3 $145.75   12.572%   $164.0737   $164.07   $164.05   $164.10  
  Purchase #4 $208.79   7.580%   $224.6163   $224.62   $224.60   $224.60  
  Purchase #5 $90.65   12.360%   $101.8543   $101.85   $101.85   $101.90  
  Purchase #6 $76.61   8.351%   $83.0077   $83.01   $83.00   $83.00  
  Purchase #7 $124.60   3.119%   $128.4863   $128.49   $128.50   $128.50  
  7 purchase totals = $752.36       $820.2582   $820.26   $820.20   $820.30