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 $209.87   11.276%   $233.5349   $233.53   $233.55   $233.50  
  Purchase #2 $245.43   12.467%   $276.0278   $276.03   $276.05   $276.00  
  Purchase #3 $20.89   12.856%   $23.5756   $23.58   $23.60   $23.60  
  Purchase #4 $95.54   10.573%   $105.6414   $105.64   $105.65   $105.60  
  Purchase #5 $69.40   12.829%   $78.3033   $78.30   $78.30   $78.30  
  Purchase #6 $118.85   6.177%   $126.1914   $126.19   $126.20   $126.20  
  Purchase #7 $133.54   3.622%   $138.3768   $138.38   $138.40   $138.40  
  7 purchase totals = $893.52       $981.6512   $981.65   $981.75   $981.60