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 $250.48   13.147%   $283.4106   $283.41   $283.40   $283.40  
  Purchase #2 $208.97   6.304%   $222.1435   $222.14   $222.15   $222.10  
  Purchase #3 $125.46   7.704%   $135.1254   $135.13   $135.15   $135.10  
  Purchase #4 $60.44   9.218%   $66.0114   $66.01   $66.00   $66.00  
  Purchase #5 $244.28   13.113%   $276.3124   $276.31   $276.30   $276.30  
  Purchase #6 $167.80   7.802%   $180.8918   $180.89   $180.90   $180.90  
  Purchase #7 $50.38   8.247%   $54.5348   $54.53   $54.55   $54.50  
  7 purchase totals = $1,107.81       $1,218.4299   $1,218.42   $1,218.45   $1,218.30