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 $33.84   13.392%   $38.3719   $38.37   $38.35   $38.40  
  Purchase #2 $196.50   6.609%   $209.4867   $209.49   $209.50   $209.50  
  Purchase #3 $103.93   9.526%   $113.8304   $113.83   $113.85   $113.80  
  Purchase #4 $212.93   13.596%   $241.88   $241.88   $241.90   $241.90  
  Purchase #5 $181.34   5.774%   $191.8106   $191.81   $191.80   $191.80  
  Purchase #6 $149.52   3.238%   $154.3615   $154.36   $154.35   $154.40  
  Purchase #7 $66.47   11.526%   $74.1313   $74.13   $74.15   $74.10  
  7 purchase totals = $944.53       $1,023.8724   $1,023.87   $1,023.90   $1,023.90