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 $16.70   10.488%   $18.4515   $18.45   $18.45   $18.50  
  Purchase #2 $16.60   3.316%   $17.1505   $17.15   $17.15   $17.20  
  Purchase #3 $232.66   3.161%   $240.0144   $240.01   $240.00   $240.00  
  Purchase #4 $18.50   3.364%   $19.1223   $19.12   $19.10   $19.10  
  Purchase #5 $72.36   11.231%   $80.4868   $80.49   $80.50   $80.50  
  Purchase #6 $8.63   13.813%   $9.8221   $9.82   $9.80   $9.80  
  Purchase #7 $91.46   9.480%   $100.1304   $100.13   $100.15   $100.10  
  7 purchase totals = $456.91       $485.1780   $485.17   $485.15   $485.20