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 $203.94   10.232%   $224.8071   $224.81   $224.80   $224.80  
  Purchase #2 $121.44   10.321%   $133.9738   $133.97   $133.95   $134.00  
  Purchase #3 $106.78   10.210%   $117.6822   $117.68   $117.70   $117.70  
  Purchase #4 $43.75   13.368%   $49.5985   $49.60   $49.60   $49.60  
  Purchase #5 $183.80   11.228%   $204.4371   $204.44   $204.45   $204.40  
  Purchase #6 $185.99   7.774%   $200.4489   $200.45   $200.45   $200.40  
  Purchase #7 $84.93   4.493%   $88.7459   $88.75   $88.75   $88.70  
  7 purchase totals = $930.63       $1,019.6935   $1,019.70   $1,019.70   $1,019.60