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 $243.41   10.739%   $269.5498   $269.55   $269.55   $269.50  
  Purchase #2 $107.78   11.308%   $119.9678   $119.97   $119.95   $120.00  
  Purchase #3 $44.59   12.661%   $50.2355   $50.24   $50.25   $50.20  
  Purchase #4 $94.28   12.629%   $106.1866   $106.19   $106.20   $106.20  
  Purchase #5 $112.64   6.485%   $119.9447   $119.94   $119.95   $119.90  
  Purchase #6 $70.59   13.497%   $80.1175   $80.12   $80.10   $80.10  
  Purchase #7 $221.50   4.625%   $231.7444   $231.74   $231.75   $231.70  
  7 purchase totals = $894.79       $977.7463   $977.75   $977.75   $977.60