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 $24.26   12.599%   $27.3165   $27.32   $27.30   $27.30  
  Purchase #2 $95.45   9.783%   $104.7879   $104.79   $104.80   $104.80  
  Purchase #3 $241.29   8.205%   $261.0878   $261.09   $261.10   $261.10  
  Purchase #4 $126.55   6.698%   $135.0263   $135.03   $135.05   $135.00  
  Purchase #5 $11.73   3.866%   $12.1835   $12.18   $12.20   $12.20  
  Purchase #6 $226.63   3.657%   $234.9179   $234.92   $234.90   $234.90  
  Purchase #7 $188.46   8.638%   $204.7392   $204.74   $204.75   $204.70  
  7 purchase totals = $914.37       $980.0591   $980.07   $980.10   $980.00