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 $250.45   12.174%   $280.9398   $280.94   $280.95   $280.90  
  Purchase #2 $128.54   8.373%   $139.3027   $139.30   $139.30   $139.30  
  Purchase #3 $216.64   6.447%   $230.6068   $230.61   $230.60   $230.60  
  Purchase #4 $226.85   11.360%   $252.6202   $252.62   $252.60   $252.60  
  Purchase #5 $49.58   12.855%   $55.9535   $55.95   $55.95   $56.00  
  Purchase #6 $169.55   4.308%   $176.8542   $176.85   $176.85   $176.90  
  Purchase #7 $219.45   12.645%   $247.1995   $247.20   $247.20   $247.20  
  7 purchase totals = $1,261.06       $1,383.4767   $1,383.47   $1,383.45   $1,383.50