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 $18.75   10.455%   $20.7103   $20.71   $20.70   $20.70  
  Purchase #2 $43.38   8.254%   $46.9606   $46.96   $46.95   $47.00  
  Purchase #3 $6.67   12.288%   $7.4896   $7.49   $7.50   $7.50  
  Purchase #4 $194.43   6.153%   $206.3933   $206.39   $206.40   $206.40  
  Purchase #5 $32.40   5.742%   $34.2604   $34.26   $34.25   $34.30  
  Purchase #6 $88.67   4.574%   $92.7258   $92.73   $92.75   $92.70  
  Purchase #7 $213.63   8.143%   $231.0259   $231.03   $231.05   $231.00  
  7 purchase totals = $597.93       $639.5659   $639.57   $639.60   $639.60