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 $50.67   5.353%   $53.3824   $53.38   $53.40   $53.40  
  Purchase #2 $195.46   3.427%   $202.1584   $202.16   $202.15   $202.20  
  Purchase #3 $5.89   7.142%   $6.3107   $6.31   $6.30   $6.30  
  Purchase #4 $173.91   3.462%   $179.9308   $179.93   $179.95   $179.90  
  Purchase #5 $91.44   5.232%   $96.2241   $96.22   $96.20   $96.20  
  Purchase #6 $161.52   10.692%   $178.7897   $178.79   $178.80   $178.80  
  Purchase #7 $250.45   9.392%   $273.9723   $273.97   $273.95   $274.00  
  7 purchase totals = $929.34       $990.7684   $990.76   $990.75   $990.80