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 $242.76   11.360%   $270.3375   $270.34   $270.35   $270.30  
  Purchase #2 $32.46   3.366%   $33.5526   $33.55   $33.55   $33.60  
  Purchase #3 $130.54   7.753%   $140.6608   $140.66   $140.65   $140.70  
  Purchase #4 $50.79   4.281%   $52.9643   $52.96   $52.95   $53.00  
  Purchase #5 $49.61   13.161%   $56.1392   $56.14   $56.15   $56.10  
  Purchase #6 $49.51   13.621%   $56.2538   $56.25   $56.25   $56.30  
  Purchase #7 $13.36   10.464%   $14.758   $14.76   $14.75   $14.80  
  7 purchase totals = $569.03       $624.6662   $624.66   $624.65   $624.80