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 $103.29   6.867%   $110.3829   $110.38   $110.40   $110.40  
  Purchase #2 $93.55   8.379%   $101.3886   $101.39   $101.40   $101.40  
  Purchase #3 $21.81   9.719%   $23.9297   $23.93   $23.95   $23.90  
  Purchase #4 $9.39   3.123%   $9.6832   $9.68   $9.70   $9.70  
  Purchase #5 $86.65   13.443%   $98.2984   $98.30   $98.30   $98.30  
  Purchase #6 $14.51   13.672%   $16.4938   $16.49   $16.50   $16.50  
  Purchase #7 $203.81   8.260%   $220.6447   $220.64   $220.65   $220.60  
  7 purchase totals = $533.01       $580.8213   $580.81   $580.90   $580.80