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 $236.99   8.573%   $257.3072   $257.31   $257.30   $257.30  
  Purchase #2 $84.26   3.293%   $87.0347   $87.03   $87.05   $87.00  
  Purchase #3 $58.88   13.608%   $66.8924   $66.89   $66.90   $66.90  
  Purchase #4 $171.93   12.704%   $193.772   $193.77   $193.75   $193.80  
  Purchase #5 $53.34   9.505%   $58.41   $58.41   $58.40   $58.40  
  Purchase #6 $133.74   11.699%   $149.3862   $149.39   $149.40   $149.40  
  Purchase #7 $18.28   5.439%   $19.2742   $19.27   $19.25   $19.30  
  7 purchase totals = $757.42       $832.0767   $832.07   $832.05   $832.10