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 $28.80   4.155%   $29.9966   $30.00   $30.00   $30.00  
  Purchase #2 $63.97   12.729%   $72.1127   $72.11   $72.10   $72.10  
  Purchase #3 $143.38   8.705%   $155.8612   $155.86   $155.85   $155.90  
  Purchase #4 $18.49   9.714%   $20.2861   $20.29   $20.30   $20.30  
  Purchase #5 $227.36   4.341%   $237.2297   $237.23   $237.25   $237.20  
  Purchase #6 $19.95   10.592%   $22.0631   $22.06   $22.05   $22.10  
  Purchase #7 $156.72   12.617%   $176.4934   $176.49   $176.50   $176.50  
  7 purchase totals = $658.67       $714.0428   $714.04   $714.05   $714.10