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 $38.51   3.576%   $39.8871   $39.89   $39.90   $39.90  
  Purchase #2 $135.92   7.477%   $146.0827   $146.08   $146.10   $146.10  
  Purchase #3 $237.64   6.127%   $252.2002   $252.20   $252.20   $252.20  
  Purchase #4 $197.82   10.271%   $218.1381   $218.14   $218.15   $218.10  
  Purchase #5 $67.34   9.547%   $73.7689   $73.77   $73.75   $73.80  
  Purchase #6 $16.84   7.546%   $18.1107   $18.11   $18.10   $18.10  
  Purchase #7 $76.38   11.420%   $85.1026   $85.10   $85.10   $85.10  
  7 purchase totals = $770.45       $833.2903   $833.29   $833.30   $833.30