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 $23.99   12.524%   $26.9945   $26.99   $27.00   $27.00  
  Purchase #2 $210.68   13.725%   $239.5958   $239.60   $239.60   $239.60  
  Purchase #3 $113.94   7.792%   $122.8182   $122.82   $122.80   $122.80  
  Purchase #4 $77.69   3.435%   $80.3587   $80.36   $80.35   $80.40  
  Purchase #5 $92.96   5.483%   $98.057   $98.06   $98.05   $98.10  
  Purchase #6 $187.68   12.159%   $210.5   $210.50   $210.50   $210.50  
  Purchase #7 $206.41   9.167%   $225.3316   $225.33   $225.35   $225.30  
  7 purchase totals = $913.35       $1,003.6558   $1,003.66   $1,003.65   $1,003.70