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 $36.36   7.849%   $39.2139   $39.21   $39.20   $39.20  
  Purchase #2 $154.54   3.628%   $160.1467   $160.15   $160.15   $160.10  
  Purchase #3 $87.83   13.311%   $99.5211   $99.52   $99.50   $99.50  
  Purchase #4 $62.64   12.860%   $70.6955   $70.70   $70.70   $70.70  
  Purchase #5 $117.33   13.562%   $133.2423   $133.24   $133.25   $133.20  
  Purchase #6 $243.78   3.480%   $252.2635   $252.26   $252.25   $252.30  
  Purchase #7 $82.96   9.603%   $90.9266   $90.93   $90.95   $90.90  
  7 purchase totals = $785.44       $846.0096   $846.01   $846.00   $845.90