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 $121.64   5.304%   $128.0918   $128.09   $128.10   $128.10  
  Purchase #2 $81.51   11.446%   $90.8396   $90.84   $90.85   $90.80  
  Purchase #3 $7.92   5.182%   $8.3304   $8.33   $8.35   $8.30  
  Purchase #4 $36.76   11.456%   $40.9712   $40.97   $40.95   $41.00  
  Purchase #5 $205.97   4.752%   $215.7577   $215.76   $215.75   $215.80  
  Purchase #6 $236.49   7.209%   $253.5386   $253.54   $253.55   $253.50  
  Purchase #7 $177.92   7.704%   $191.627   $191.63   $191.65   $191.60  
  7 purchase totals = $868.21       $929.1563   $929.16   $929.20   $929.10