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 $115.42   12.694%   $130.0714   $130.07   $130.05   $130.10  
  Purchase #2 $197.78   4.273%   $206.2311   $206.23   $206.25   $206.20  
  Purchase #3 $16.29   11.264%   $18.1249   $18.12   $18.10   $18.10  
  Purchase #4 $121.87   11.871%   $136.3372   $136.34   $136.35   $136.30  
  Purchase #5 $210.79   3.305%   $217.7566   $217.76   $217.75   $217.80  
  Purchase #6 $58.70   6.703%   $62.6347   $62.63   $62.65   $62.60  
  Purchase #7 $159.47   4.633%   $166.8582   $166.86   $166.85   $166.90  
  7 purchase totals = $880.32       $938.0141   $938.01   $938.00   $938.00