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 $17.64   13.341%   $19.9934   $19.99   $20.00   $20.00  
  Purchase #2 $29.62   10.860%   $32.8367   $32.84   $32.85   $32.80  
  Purchase #3 $206.39   3.618%   $213.8572   $213.86   $213.85   $213.90  
  Purchase #4 $5.87   5.804%   $6.2107   $6.21   $6.20   $6.20  
  Purchase #5 $172.95   12.209%   $194.0655   $194.07   $194.05   $194.10  
  Purchase #6 $40.49   5.167%   $42.5821   $42.58   $42.60   $42.60  
  Purchase #7 $170.87   12.619%   $192.4321   $192.43   $192.45   $192.40  
  7 purchase totals = $643.83       $701.9777   $701.98   $702.00   $702.00