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 $236.50   11.235%   $263.0708   $263.07   $263.05   $263.10  
  Purchase #2 $217.57   10.299%   $239.9775   $239.98   $240.00   $240.00  
  Purchase #3 $29.39   12.839%   $33.1634   $33.16   $33.15   $33.20  
  Purchase #4 $241.55   13.502%   $274.1641   $274.16   $274.15   $274.20  
  Purchase #5 $8.85   6.542%   $9.429   $9.43   $9.45   $9.40  
  Purchase #6 $21.83   5.410%   $23.011   $23.01   $23.00   $23.00  
  Purchase #7 $235.31   4.692%   $246.3507   $246.35   $246.35   $246.40  
  7 purchase totals = $991.00       $1,089.1665   $1,089.16   $1,089.15   $1,089.30