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 $154.26   8.406%   $167.2271   $167.23   $167.25   $167.20  
  Purchase #2 $65.37   13.231%   $74.0191   $74.02   $74.00   $74.00  
  Purchase #3 $18.32   9.669%   $20.0914   $20.09   $20.10   $20.10  
  Purchase #4 $88.73   9.714%   $97.3492   $97.35   $97.35   $97.30  
  Purchase #5 $107.63   11.155%   $119.6361   $119.64   $119.65   $119.60  
  Purchase #6 $118.47   4.506%   $123.8083   $123.81   $123.80   $123.80  
  Purchase #7 $35.85   5.696%   $37.892   $37.89   $37.90   $37.90  
  7 purchase totals = $588.63       $640.0232   $640.03   $640.05   $639.90