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 $227.29   11.579%   $253.6079   $253.61   $253.60   $253.60  
  Purchase #2 $183.81   13.143%   $207.9681   $207.97   $207.95   $208.00  
  Purchase #3 $20.40   4.456%   $21.309   $21.31   $21.30   $21.30  
  Purchase #4 $199.91   7.782%   $215.467   $215.47   $215.45   $215.50  
  Purchase #5 $164.92   10.274%   $181.8639   $181.86   $181.85   $181.90  
  Purchase #6 $150.79   8.362%   $163.3991   $163.40   $163.40   $163.40  
  Purchase #7 $45.64   4.474%   $47.6819   $47.68   $47.70   $47.70  
  7 purchase totals = $992.76       $1,091.2969   $1,091.30   $1,091.25   $1,091.40