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 $10.76   9.349%   $11.766   $11.77   $11.75   $11.80  
  Purchase #2 $7.58   3.238%   $7.8254   $7.83   $7.85   $7.80  
  Purchase #3 $14.85   7.237%   $15.9247   $15.92   $15.90   $15.90  
  Purchase #4 $49.97   5.466%   $52.7014   $52.70   $52.70   $52.70  
  Purchase #5 $15.87   11.631%   $17.7158   $17.72   $17.70   $17.70  
  Purchase #6 $106.81   12.383%   $120.0363   $120.04   $120.05   $120.00  
  Purchase #7 $80.82   3.664%   $83.7812   $83.78   $83.80   $83.80  
  7 purchase totals = $286.66       $309.7508   $309.76   $309.75   $309.70