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 $249.44   8.639%   $270.9891   $270.99   $271.00   $271.00  
  Purchase #2 $188.80   6.364%   $200.8152   $200.82   $200.80   $200.80  
  Purchase #3 $23.72   12.292%   $26.6357   $26.64   $26.65   $26.60  
  Purchase #4 $105.69   11.290%   $117.6224   $117.62   $117.60   $117.60  
  Purchase #5 $214.50   5.264%   $225.7913   $225.79   $225.80   $225.80  
  Purchase #6 $98.84   10.461%   $109.1797   $109.18   $109.20   $109.20  
  Purchase #7 $57.50   9.605%   $63.0229   $63.02   $63.00   $63.00  
  7 purchase totals = $938.49       $1,014.0563   $1,014.06   $1,014.05   $1,014.00