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 $249.50   8.731%   $271.2838   $271.28   $271.30   $271.30  
  Purchase #2 $130.92   7.184%   $140.3253   $140.33   $140.35   $140.30  
  Purchase #3 $175.62   7.553%   $188.8846   $188.88   $188.90   $188.90  
  Purchase #4 $85.29   6.589%   $90.9098   $90.91   $90.90   $90.90  
  Purchase #5 $211.87   12.771%   $238.9279   $238.93   $238.95   $238.90  
  Purchase #6 $221.29   10.548%   $244.6317   $244.63   $244.65   $244.60  
  Purchase #7 $24.66   9.744%   $27.0629   $27.06   $27.05   $27.10  
  7 purchase totals = $1,099.15       $1,202.0260   $1,202.02   $1,202.10   $1,202.00