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 $7.45   5.779%   $7.8805   $7.88   $7.90   $7.90  
  Purchase #2 $17.59   13.586%   $19.9798   $19.98   $20.00   $20.00  
  Purchase #3 $91.43   6.553%   $97.4214   $97.42   $97.40   $97.40  
  Purchase #4 $93.62   5.827%   $99.0752   $99.08   $99.10   $99.10  
  Purchase #5 $13.87   4.221%   $14.4555   $14.46   $14.45   $14.50  
  Purchase #6 $181.93   10.493%   $201.0199   $201.02   $201.00   $201.00  
  Purchase #7 $76.34   12.455%   $85.8481   $85.85   $85.85   $85.80  
  7 purchase totals = $482.23       $525.6804   $525.69   $525.70   $525.70