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 $35.26   10.693%   $39.0304   $39.03   $39.05   $39.00  
  Purchase #2 $233.38   13.593%   $265.1033   $265.10   $265.10   $265.10  
  Purchase #3 $58.83   12.660%   $66.2779   $66.28   $66.30   $66.30  
  Purchase #4 $17.47   13.110%   $19.7603   $19.76   $19.75   $19.80  
  Purchase #5 $218.57   6.386%   $232.5279   $232.53   $232.55   $232.50  
  Purchase #6 $232.25   4.388%   $242.4411   $242.44   $242.45   $242.40  
  Purchase #7 $156.96   9.630%   $172.0752   $172.08   $172.10   $172.10  
  7 purchase totals = $952.72       $1,037.2161   $1,037.22   $1,037.30   $1,037.20