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 $78.70   13.500%   $89.3245   $89.32   $89.30   $89.30  
  Purchase #2 $7.46   6.222%   $7.9242   $7.92   $7.90   $7.90  
  Purchase #3 $203.47   10.672%   $225.1843   $225.18   $225.20   $225.20  
  Purchase #4 $5.30   5.124%   $5.5716   $5.57   $5.55   $5.60  
  Purchase #5 $61.43   12.244%   $68.9515   $68.95   $68.95   $69.00  
  Purchase #6 $40.42   10.743%   $44.7623   $44.76   $44.75   $44.80  
  Purchase #7 $243.57   10.411%   $268.9281   $268.93   $268.95   $268.90  
  7 purchase totals = $640.35       $710.6465   $710.63   $710.60   $710.70