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 $167.78   10.642%   $185.6351   $185.64   $185.65   $185.60  
  Purchase #2 $156.98   6.116%   $166.5809   $166.58   $166.60   $166.60  
  Purchase #3 $220.90   12.860%   $249.3077   $249.31   $249.30   $249.30  
  Purchase #4 $38.30   5.386%   $40.3628   $40.36   $40.35   $40.40  
  Purchase #5 $52.66   8.378%   $57.0719   $57.07   $57.05   $57.10  
  Purchase #6 $20.95   9.114%   $22.8594   $22.86   $22.85   $22.90  
  Purchase #7 $97.25   11.482%   $108.4162   $108.42   $108.40   $108.40  
  7 purchase totals = $754.82       $830.2340   $830.24   $830.20   $830.30