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 $245.62   8.104%   $265.525   $265.53   $265.55   $265.50  
  Purchase #2 $117.60   9.298%   $128.5344   $128.53   $128.55   $128.50  
  Purchase #3 $75.80   11.659%   $84.6375   $84.64   $84.65   $84.60  
  Purchase #4 $17.65   11.626%   $19.702   $19.70   $19.70   $19.70  
  Purchase #5 $82.72   9.187%   $90.3195   $90.32   $90.30   $90.30  
  Purchase #6 $205.40   11.172%   $228.3473   $228.35   $228.35   $228.30  
  Purchase #7 $232.92   12.437%   $261.8883   $261.89   $261.90   $261.90  
  7 purchase totals = $977.71       $1,078.9540   $1,078.96   $1,079.00   $1,078.80