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 $39.95   5.762%   $42.2519   $42.25   $42.25   $42.30  
  Purchase #2 $67.89   6.755%   $72.476   $72.48   $72.50   $72.50  
  Purchase #3 $198.78   10.705%   $220.0594   $220.06   $220.05   $220.10  
  Purchase #4 $174.44   13.598%   $198.1604   $198.16   $198.15   $198.20  
  Purchase #5 $119.61   7.489%   $128.5676   $128.57   $128.55   $128.60  
  Purchase #6 $57.51   7.277%   $61.695   $61.70   $61.70   $61.70  
  Purchase #7 $110.86   10.377%   $122.3639   $122.36   $122.35   $122.40  
  7 purchase totals = $769.04       $845.5742   $845.58   $845.55   $845.80