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 $15.75   13.559%   $17.8855   $17.89   $17.90   $17.90  
  Purchase #2 $236.42   7.500%   $254.1515   $254.15   $254.15   $254.20  
  Purchase #3 $71.98   3.338%   $74.3827   $74.38   $74.40   $74.40  
  Purchase #4 $122.78   9.521%   $134.4699   $134.47   $134.45   $134.50  
  Purchase #5 $105.80   3.618%   $109.6278   $109.63   $109.65   $109.60  
  Purchase #6 $62.57   7.696%   $67.3854   $67.39   $67.40   $67.40  
  Purchase #7 $32.79   6.479%   $34.9145   $34.91   $34.90   $34.90  
  7 purchase totals = $648.09       $692.8173   $692.82   $692.85   $692.90