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 $170.57   11.284%   $189.8171   $189.82   $189.80   $189.80  
  Purchase #2 $239.68   8.825%   $260.8318   $260.83   $260.85   $260.80  
  Purchase #3 $188.57   5.410%   $198.7716   $198.77   $198.75   $198.80  
  Purchase #4 $116.55   7.423%   $125.2015   $125.20   $125.20   $125.20  
  Purchase #5 $97.69   4.528%   $102.1134   $102.11   $102.10   $102.10  
  Purchase #6 $242.40   12.434%   $272.54   $272.54   $272.55   $272.50  
  Purchase #7 $33.57   9.258%   $36.6779   $36.68   $36.70   $36.70  
  7 purchase totals = $1,089.03       $1,185.9533   $1,185.95   $1,185.95   $1,185.90