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 $246.58   12.716%   $277.9351   $277.94   $277.95   $277.90  
  Purchase #2 $121.82   11.595%   $135.945   $135.95   $135.95   $135.90  
  Purchase #3 $231.59   9.163%   $252.8106   $252.81   $252.80   $252.80  
  Purchase #4 $223.64   3.184%   $230.7607   $230.76   $230.75   $230.80  
  Purchase #5 $85.72   9.112%   $93.5308   $93.53   $93.55   $93.50  
  Purchase #6 $212.28   11.246%   $236.153   $236.15   $236.15   $236.20  
  Purchase #7 $129.38   3.404%   $133.7841   $133.78   $133.80   $133.80  
  7 purchase totals = $1,251.01       $1,360.9193   $1,360.92   $1,360.95   $1,360.90