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 $19.51   7.156%   $20.9061   $20.91   $20.90   $20.90  
  Purchase #2 $13.95   13.785%   $15.873   $15.87   $15.85   $15.90  
  Purchase #3 $131.65   6.209%   $139.8241   $139.82   $139.80   $139.80  
  Purchase #4 $141.34   5.480%   $149.0854   $149.09   $149.10   $149.10  
  Purchase #5 $5.55   7.435%   $5.9626   $5.96   $5.95   $6.00  
  Purchase #6 $202.78   9.128%   $221.2898   $221.29   $221.30   $221.30  
  Purchase #7 $56.50   13.331%   $64.032   $64.03   $64.05   $64.00  
  7 purchase totals = $571.28       $616.9730   $616.97   $616.95   $617.00