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 $127.42   9.116%   $139.0356   $139.04   $139.05   $139.00  
  Purchase #2 $144.49   3.863%   $150.0716   $150.07   $150.05   $150.10  
  Purchase #3 $198.79   10.581%   $219.824   $219.82   $219.80   $219.80  
  Purchase #4 $128.37   7.732%   $138.2956   $138.30   $138.30   $138.30  
  Purchase #5 $59.61   6.492%   $63.4799   $63.48   $63.50   $63.50  
  Purchase #6 $50.71   12.595%   $57.0969   $57.10   $57.10   $57.10  
  Purchase #7 $5.71   11.270%   $6.3535   $6.35   $6.35   $6.40  
  7 purchase totals = $715.10       $774.1571   $774.16   $774.15   $774.20