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 $166.35   12.172%   $186.5981   $186.60   $186.60   $186.60  
  Purchase #2 $159.86   13.760%   $181.8567   $181.86   $181.85   $181.90  
  Purchase #3 $196.70   4.726%   $205.996   $206.00   $206.00   $206.00  
  Purchase #4 $70.40   13.502%   $79.9054   $79.91   $79.90   $79.90  
  Purchase #5 $33.68   9.356%   $36.8311   $36.83   $36.85   $36.80  
  Purchase #6 $199.97   3.697%   $207.3629   $207.36   $207.35   $207.40  
  Purchase #7 $77.75   7.309%   $83.4327   $83.43   $83.45   $83.40  
  7 purchase totals = $904.71       $981.9829   $981.99   $982.00   $982.00