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 $179.28   13.718%   $203.8736   $203.87   $203.85   $203.90  
  Purchase #2 $28.81   12.840%   $32.5092   $32.51   $32.50   $32.50  
  Purchase #3 $30.47   12.556%   $34.2958   $34.30   $34.30   $34.30  
  Purchase #4 $124.30   5.697%   $131.3814   $131.38   $131.40   $131.40  
  Purchase #5 $250.44   9.818%   $275.0282   $275.03   $275.05   $275.00  
  Purchase #6 $71.44   4.209%   $74.4469   $74.45   $74.45   $74.40  
  Purchase #7 $205.98   11.572%   $229.816   $229.82   $229.80   $229.80  
  7 purchase totals = $890.72       $981.3511   $981.36   $981.35   $981.30