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 $248.26   9.572%   $272.0234   $272.02   $272.00   $272.00  
  Purchase #2 $216.82   10.616%   $239.8376   $239.84   $239.85   $239.80  
  Purchase #3 $92.65   13.113%   $104.7992   $104.80   $104.80   $104.80  
  Purchase #4 $236.43   8.811%   $257.2618   $257.26   $257.25   $257.30  
  Purchase #5 $131.40   13.749%   $149.4662   $149.47   $149.45   $149.50  
  Purchase #6 $31.64   9.268%   $34.5724   $34.57   $34.55   $34.60  
  Purchase #7 $228.38   4.475%   $238.6   $238.60   $238.60   $238.60  
  7 purchase totals = $1,185.58       $1,296.5606   $1,296.56   $1,296.50   $1,296.60