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 $248.35   11.511%   $276.9376   $276.94   $276.95   $276.90  
  Purchase #2 $181.90   10.866%   $201.6653   $201.67   $201.65   $201.70  
  Purchase #3 $219.67   10.178%   $242.028   $242.03   $242.05   $242.00  
  Purchase #4 $56.62   5.810%   $59.9096   $59.91   $59.90   $59.90  
  Purchase #5 $20.83   10.553%   $23.0282   $23.03   $23.05   $23.00  
  Purchase #6 $67.59   5.791%   $71.5041   $71.50   $71.50   $71.50  
  Purchase #7 $199.72   7.293%   $214.2856   $214.29   $214.30   $214.30  
  7 purchase totals = $994.68       $1,089.3584   $1,089.37   $1,089.40   $1,089.30