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 $6.40   13.513%   $7.2648   $7.26   $7.25   $7.30  
  Purchase #2 $104.48   7.866%   $112.6984   $112.70   $112.70   $112.70  
  Purchase #3 $202.85   8.337%   $219.7616   $219.76   $219.75   $219.80  
  Purchase #4 $60.42   11.527%   $67.3846   $67.38   $67.40   $67.40  
  Purchase #5 $44.61   11.405%   $49.6978   $49.70   $49.70   $49.70  
  Purchase #6 $44.69   11.487%   $49.8235   $49.82   $49.80   $49.80  
  Purchase #7 $28.93   9.289%   $31.6173   $31.62   $31.60   $31.60  
  7 purchase totals = $492.38       $538.2480   $538.24   $538.20   $538.30