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 $247.43   11.784%   $276.5872   $276.59   $276.60   $276.60  
  Purchase #2 $229.67   12.394%   $258.1353   $258.14   $258.15   $258.10  
  Purchase #3 $191.52   6.430%   $203.8347   $203.83   $203.85   $203.80  
  Purchase #4 $107.38   3.659%   $111.309   $111.31   $111.30   $111.30  
  Purchase #5 $58.30   8.262%   $63.1167   $63.12   $63.10   $63.10  
  Purchase #6 $221.81   13.666%   $252.1226   $252.12   $252.10   $252.10  
  Purchase #7 $221.56   13.166%   $250.7306   $250.73   $250.75   $250.70  
  7 purchase totals = $1,277.67       $1,415.8361   $1,415.84   $1,415.85   $1,415.70