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 $112.25   11.131%   $124.7445   $124.74   $124.75   $124.70  
  Purchase #2 $29.67   9.802%   $32.5783   $32.58   $32.60   $32.60  
  Purchase #3 $28.54   4.488%   $29.8209   $29.82   $29.80   $29.80  
  Purchase #4 $64.34   6.381%   $68.4455   $68.45   $68.45   $68.40  
  Purchase #5 $226.94   6.829%   $242.4377   $242.44   $242.45   $242.40  
  Purchase #6 $7.49   11.379%   $8.3423   $8.34   $8.35   $8.30  
  Purchase #7 $87.25   6.591%   $93.0006   $93.00   $93.00   $93.00  
  7 purchase totals = $556.48       $599.3698   $599.37   $599.40   $599.20