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 $129.43   9.874%   $142.2099   $142.21   $142.20   $142.20  
  Purchase #2 $7.25   10.405%   $8.0044   $8.00   $8.00   $8.00  
  Purchase #3 $189.86   9.367%   $207.6442   $207.64   $207.65   $207.60  
  Purchase #4 $157.25   8.640%   $170.8364   $170.84   $170.85   $170.80  
  Purchase #5 $83.91   8.569%   $91.1002   $91.10   $91.10   $91.10  
  Purchase #6 $137.38   3.148%   $141.7047   $141.70   $141.70   $141.70  
  Purchase #7 $161.63   9.793%   $177.4584   $177.46   $177.45   $177.50  
  7 purchase totals = $866.71       $938.9582   $938.95   $938.95   $938.90