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 $250.55   12.233%   $281.1998   $281.20   $281.20   $281.20  
  Purchase #2 $211.45   10.644%   $233.9567   $233.96   $233.95   $234.00  
  Purchase #3 $216.81   6.153%   $230.1503   $230.15   $230.15   $230.20  
  Purchase #4 $89.31   7.750%   $96.2315   $96.23   $96.25   $96.20  
  Purchase #5 $154.59   9.120%   $168.6886   $168.69   $168.70   $168.70  
  Purchase #6 $165.34   11.789%   $184.8319   $184.83   $184.85   $184.80  
  Purchase #7 $162.99   7.426%   $175.0936   $175.09   $175.10   $175.10  
  7 purchase totals = $1,251.04       $1,370.1524   $1,370.15   $1,370.20   $1,370.20