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 $133.28   10.484%   $147.2531   $147.25   $147.25   $147.30  
  Purchase #2 $72.49   10.564%   $80.1478   $80.15   $80.15   $80.10  
  Purchase #3 $155.96   5.311%   $164.243   $164.24   $164.25   $164.20  
  Purchase #4 $53.25   5.843%   $56.3614   $56.36   $56.35   $56.40  
  Purchase #5 $53.30   7.499%   $57.297   $57.30   $57.30   $57.30  
  Purchase #6 $57.58   13.287%   $65.2307   $65.23   $65.25   $65.20  
  Purchase #7 $69.48   5.384%   $73.2208   $73.22   $73.20   $73.20  
  7 purchase totals = $595.34       $643.7538   $643.75   $643.75   $643.70