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 $128.33   8.351%   $139.0468   $139.05   $139.05   $139.00  
  Purchase #2 $75.29   6.370%   $80.086   $80.09   $80.10   $80.10  
  Purchase #3 $178.80   13.759%   $203.4011   $203.40   $203.40   $203.40  
  Purchase #4 $6.69   8.246%   $7.2417   $7.24   $7.25   $7.20  
  Purchase #5 $177.59   9.509%   $194.477   $194.48   $194.50   $194.50  
  Purchase #6 $143.86   8.677%   $156.3427   $156.34   $156.35   $156.30  
  Purchase #7 $99.75   3.347%   $103.0886   $103.09   $103.10   $103.10  
  7 purchase totals = $810.31       $883.6839   $883.69   $883.75   $883.60