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 $97.69   5.548%   $103.1098   $103.11   $103.10   $103.10  
  Purchase #2 $203.86   9.307%   $222.8333   $222.83   $222.85   $222.80  
  Purchase #3 $181.66   11.175%   $201.9605   $201.96   $201.95   $202.00  
  Purchase #4 $126.55   9.745%   $138.8823   $138.88   $138.90   $138.90  
  Purchase #5 $109.51   9.319%   $119.7152   $119.72   $119.70   $119.70  
  Purchase #6 $30.29   12.656%   $34.1235   $34.12   $34.10   $34.10  
  Purchase #7 $34.81   13.128%   $39.3799   $39.38   $39.40   $39.40  
  7 purchase totals = $784.37       $860.0045   $860.00   $860.00   $860.00