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 $29.50   3.113%   $30.4183   $30.42   $30.40   $30.40  
  Purchase #2 $156.45   9.212%   $170.8622   $170.86   $170.85   $170.90  
  Purchase #3 $8.73   8.701%   $9.4896   $9.49   $9.50   $9.50  
  Purchase #4 $192.84   13.745%   $219.3459   $219.35   $219.35   $219.30  
  Purchase #5 $158.95   4.682%   $166.392   $166.39   $166.40   $166.40  
  Purchase #6 $131.93   5.560%   $139.2653   $139.27   $139.25   $139.30  
  Purchase #7 $172.45   7.473%   $185.3372   $185.34   $185.35   $185.30  
  7 purchase totals = $850.85       $921.1105   $921.12   $921.10   $921.10