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 $44.47   6.781%   $47.4855   $47.49   $47.50   $47.50  
  Purchase #2 $228.96   11.807%   $255.9933   $255.99   $256.00   $256.00  
  Purchase #3 $239.63   8.281%   $259.4738   $259.47   $259.45   $259.50  
  Purchase #4 $209.45   12.224%   $235.0532   $235.05   $235.05   $235.10  
  Purchase #5 $25.64   10.501%   $28.3325   $28.33   $28.35   $28.30  
  Purchase #6 $140.34   7.613%   $151.0241   $151.02   $151.00   $151.00  
  Purchase #7 $15.58   4.759%   $16.3215   $16.32   $16.30   $16.30  
  7 purchase totals = $904.07       $993.6839   $993.67   $993.65   $993.70