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 $12.77   4.791%   $13.3818   $13.38   $13.40   $13.40  
  Purchase #2 $79.66   4.739%   $83.4351   $83.44   $83.45   $83.40  
  Purchase #3 $15.94   11.558%   $17.7823   $17.78   $17.80   $17.80  
  Purchase #4 $243.45   10.153%   $268.1675   $268.17   $268.15   $268.20  
  Purchase #5 $14.61   9.490%   $15.9965   $16.00   $16.00   $16.00  
  Purchase #6 $142.48   11.492%   $158.8538   $158.85   $158.85   $158.90  
  Purchase #7 $183.37   12.444%   $206.1886   $206.19   $206.20   $206.20  
  7 purchase totals = $692.28       $763.8056   $763.81   $763.85   $763.90