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 $77.91   4.741%   $81.6037   $81.60   $81.60   $81.60  
  Purchase #2 $47.88   8.691%   $52.0413   $52.04   $52.05   $52.00  
  Purchase #3 $34.60   13.842%   $39.3893   $39.39   $39.40   $39.40  
  Purchase #4 $7.87   10.506%   $8.6968   $8.70   $8.70   $8.70  
  Purchase #5 $40.95   10.426%   $45.2194   $45.22   $45.20   $45.20  
  Purchase #6 $98.62   12.541%   $110.9879   $110.99   $111.00   $111.00  
  Purchase #7 $237.83   12.542%   $267.6586   $267.66   $267.65   $267.70  
  7 purchase totals = $545.66       $605.5970   $605.60   $605.60   $605.60