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 $88.93   10.703%   $98.4482   $98.45   $98.45   $98.40  
  Purchase #2 $96.34   4.729%   $100.8959   $100.90   $100.90   $100.90  
  Purchase #3 $207.98   9.550%   $227.8421   $227.84   $227.85   $227.80  
  Purchase #4 $84.69   9.220%   $92.4984   $92.50   $92.50   $92.50  
  Purchase #5 $202.76   8.127%   $219.2383   $219.24   $219.25   $219.20  
  Purchase #6 $65.88   13.587%   $74.8311   $74.83   $74.85   $74.80  
  Purchase #7 $170.31   9.426%   $186.3634   $186.36   $186.35   $186.40  
  7 purchase totals = $916.89       $1,000.1174   $1,000.12   $1,000.15   $1,000.00