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 $155.80   10.218%   $171.7196   $171.72   $171.70   $171.70  
  Purchase #2 $143.29   6.192%   $152.1625   $152.16   $152.15   $152.20  
  Purchase #3 $15.93   7.381%   $17.1058   $17.11   $17.10   $17.10  
  Purchase #4 $37.47   13.748%   $42.6214   $42.62   $42.60   $42.60  
  Purchase #5 $30.67   4.114%   $31.9318   $31.93   $31.95   $31.90  
  Purchase #6 $35.90   10.467%   $39.6577   $39.66   $39.65   $39.70  
  Purchase #7 $120.36   3.306%   $124.3391   $124.34   $124.35   $124.30  
  7 purchase totals = $539.42       $579.5379   $579.54   $579.50   $579.50