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 $83.75   3.491%   $86.6737   $86.67   $86.65   $86.70  
  Purchase #2 $156.25   13.874%   $177.9281   $177.93   $177.95   $177.90  
  Purchase #3 $90.59   6.619%   $96.5862   $96.59   $96.60   $96.60  
  Purchase #4 $153.32   5.368%   $161.5502   $161.55   $161.55   $161.60  
  Purchase #5 $78.81   6.621%   $84.028   $84.03   $84.05   $84.00  
  Purchase #6 $234.91   12.635%   $264.5909   $264.59   $264.60   $264.60  
  Purchase #7 $51.29   11.542%   $57.2099   $57.21   $57.20   $57.20  
  7 purchase totals = $848.92       $928.5670   $928.57   $928.60   $928.60