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 $181.40   4.538%   $189.6319   $189.63   $189.65   $189.60  
  Purchase #2 $149.31   6.312%   $158.7344   $158.73   $158.75   $158.70  
  Purchase #3 $10.69   4.110%   $11.1294   $11.13   $11.15   $11.10  
  Purchase #4 $129.50   8.694%   $140.7587   $140.76   $140.75   $140.80  
  Purchase #5 $96.79   12.414%   $108.8055   $108.81   $108.80   $108.80  
  Purchase #6 $20.98   3.616%   $21.7386   $21.74   $21.75   $21.70  
  Purchase #7 $31.33   4.591%   $32.7684   $32.77   $32.75   $32.80  
  7 purchase totals = $620.00       $663.5669   $663.57   $663.60   $663.50