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 $18.96   5.672%   $20.0354   $20.04   $20.05   $20.00  
  Purchase #2 $38.43   9.173%   $41.9552   $41.96   $41.95   $42.00  
  Purchase #3 $86.26   3.469%   $89.2524   $89.25   $89.25   $89.30  
  Purchase #4 $161.99   4.680%   $169.5711   $169.57   $169.55   $169.60  
  Purchase #5 $127.42   8.321%   $138.0226   $138.02   $138.00   $138.00  
  Purchase #6 $125.60   3.231%   $129.6581   $129.66   $129.65   $129.70  
  Purchase #7 $98.25   7.570%   $105.6875   $105.69   $105.70   $105.70  
  7 purchase totals = $656.91       $694.1823   $694.19   $694.15   $694.30