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 $21.30   8.864%   $23.188   $23.19   $23.20   $23.20  
  Purchase #2 $20.81   10.648%   $23.0258   $23.03   $23.05   $23.00  
  Purchase #3 $241.55   4.436%   $252.2652   $252.27   $252.25   $252.30  
  Purchase #4 $83.61   9.112%   $91.2285   $91.23   $91.25   $91.20  
  Purchase #5 $33.25   13.384%   $37.7002   $37.70   $37.70   $37.70  
  Purchase #6 $79.34   7.605%   $85.3738   $85.37   $85.35   $85.40  
  Purchase #7 $118.75   10.691%   $131.4456   $131.45   $131.45   $131.40  
  7 purchase totals = $598.61       $644.2271   $644.24   $644.25   $644.20