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 $27.88   7.595%   $29.9975   $30.00   $30.00   $30.00  
  Purchase #2 $100.78   7.296%   $108.1329   $108.13   $108.15   $108.10  
  Purchase #3 $217.35   4.298%   $226.6917   $226.69   $226.70   $226.70  
  Purchase #4 $210.48   13.365%   $238.6107   $238.61   $238.60   $238.60  
  Purchase #5 $13.50   13.450%   $15.3158   $15.32   $15.30   $15.30  
  Purchase #6 $137.64   9.164%   $150.2533   $150.25   $150.25   $150.30  
  Purchase #7 $159.61   8.811%   $173.6732   $173.67   $173.65   $173.70  
  7 purchase totals = $867.24       $942.6751   $942.67   $942.65   $942.70