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 $84.61   7.860%   $91.2603   $91.26   $91.25   $91.30  
  Purchase #2 $91.41   3.438%   $94.5527   $94.55   $94.55   $94.60  
  Purchase #3 $75.98   8.623%   $82.5318   $82.53   $82.55   $82.50  
  Purchase #4 $118.92   12.395%   $133.6601   $133.66   $133.65   $133.70  
  Purchase #5 $243.85   4.807%   $255.5719   $255.57   $255.55   $255.60  
  Purchase #6 $104.69   12.818%   $118.1092   $118.11   $118.10   $118.10  
  Purchase #7 $190.45   13.248%   $215.6808   $215.68   $215.70   $215.70  
  7 purchase totals = $909.91       $991.3668   $991.36   $991.35   $991.50