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 $13.37   12.752%   $15.0749   $15.07   $15.05   $15.10  
  Purchase #2 $25.85   11.466%   $28.814   $28.81   $28.80   $28.80  
  Purchase #3 $218.98   7.714%   $235.8721   $235.87   $235.85   $235.90  
  Purchase #4 $87.65   4.505%   $91.5986   $91.60   $91.60   $91.60  
  Purchase #5 $83.87   4.806%   $87.9008   $87.90   $87.90   $87.90  
  Purchase #6 $88.70   7.466%   $95.3223   $95.32   $95.30   $95.30  
  Purchase #7 $79.41   5.477%   $83.7593   $83.76   $83.75   $83.80  
  7 purchase totals = $597.83       $638.3420   $638.33   $638.25   $638.40