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 $230.53   13.439%   $261.5109   $261.51   $261.50   $261.50  
  Purchase #2 $152.96   13.383%   $173.4306   $173.43   $173.45   $173.40  
  Purchase #3 $8.65   13.827%   $9.846   $9.85   $9.85   $9.80  
  Purchase #4 $101.27   4.329%   $105.654   $105.65   $105.65   $105.70  
  Purchase #5 $199.47   6.466%   $212.3677   $212.37   $212.35   $212.40  
  Purchase #6 $127.74   9.482%   $139.8523   $139.85   $139.85   $139.90  
  Purchase #7 $123.79   12.147%   $138.8268   $138.83   $138.85   $138.80  
  7 purchase totals = $944.41       $1,041.4883   $1,041.49   $1,041.50   $1,041.50