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 $25.41   9.315%   $27.7769   $27.78   $27.80   $27.80  
  Purchase #2 $89.93   8.371%   $97.458   $97.46   $97.45   $97.50  
  Purchase #3 $230.95   10.577%   $255.3776   $255.38   $255.40   $255.40  
  Purchase #4 $99.63   10.394%   $109.9855   $109.99   $110.00   $110.00  
  Purchase #5 $119.45   3.669%   $123.8326   $123.83   $123.85   $123.80  
  Purchase #6 $102.95   9.409%   $112.6366   $112.64   $112.65   $112.60  
  Purchase #7 $52.40   9.717%   $57.4917   $57.49   $57.50   $57.50  
  7 purchase totals = $720.72       $784.5589   $784.57   $784.65   $784.60