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 $120.70   12.270%   $135.5099   $135.51   $135.50   $135.50  
  Purchase #2 $182.70   5.610%   $192.9495   $192.95   $192.95   $192.90  
  Purchase #3 $241.40   4.466%   $252.1809   $252.18   $252.20   $252.20  
  Purchase #4 $68.30   6.169%   $72.5134   $72.51   $72.50   $72.50  
  Purchase #5 $125.38   8.818%   $136.436   $136.44   $136.45   $136.40  
  Purchase #6 $186.42   5.865%   $197.3535   $197.35   $197.35   $197.40  
  Purchase #7 $68.69   11.370%   $76.5001   $76.50   $76.50   $76.50  
  7 purchase totals = $993.59       $1,063.4433   $1,063.44   $1,063.45   $1,063.40