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 $244.47   6.292%   $259.8521   $259.85   $259.85   $259.90  
  Purchase #2 $14.34   13.741%   $16.3105   $16.31   $16.30   $16.30  
  Purchase #3 $101.51   13.791%   $115.5092   $115.51   $115.50   $115.50  
  Purchase #4 $216.75   5.160%   $227.9343   $227.93   $227.95   $227.90  
  Purchase #5 $117.56   13.392%   $133.3036   $133.30   $133.30   $133.30  
  Purchase #6 $30.67   9.587%   $33.6103   $33.61   $33.60   $33.60  
  Purchase #7 $188.67   13.832%   $214.7668   $214.77   $214.75   $214.80  
  7 purchase totals = $913.97       $1,001.2868   $1,001.28   $1,001.25   $1,001.30