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 $6.79   4.491%   $7.0949   $7.09   $7.10   $7.10  
  Purchase #2 $16.62   8.472%   $18.028   $18.03   $18.05   $18.00  
  Purchase #3 $20.49   9.120%   $22.3587   $22.36   $22.35   $22.40  
  Purchase #4 $112.77   10.376%   $124.471   $124.47   $124.45   $124.50  
  Purchase #5 $135.74   12.210%   $152.3139   $152.31   $152.30   $152.30  
  Purchase #6 $220.82   8.283%   $239.1105   $239.11   $239.10   $239.10  
  Purchase #7 $41.99   4.341%   $43.8128   $43.81   $43.80   $43.80  
  7 purchase totals = $555.22       $607.1898   $607.18   $607.15   $607.20