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 $17.51   3.558%   $18.133   $18.13   $18.15   $18.10  
  Purchase #2 $61.95   11.600%   $69.1362   $69.14   $69.15   $69.10  
  Purchase #3 $15.48   10.304%   $17.0751   $17.08   $17.10   $17.10  
  Purchase #4 $10.69   7.349%   $11.4756   $11.48   $11.50   $11.50  
  Purchase #5 $120.98   11.345%   $134.7052   $134.71   $134.70   $134.70  
  Purchase #6 $132.83   4.341%   $138.5962   $138.60   $138.60   $138.60  
  Purchase #7 $234.41   7.142%   $251.1516   $251.15   $251.15   $251.20  
  7 purchase totals = $593.85       $640.2729   $640.29   $640.35   $640.30