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 $130.33   3.744%   $135.2096   $135.21   $135.20   $135.20  
  Purchase #2 $234.77   7.370%   $252.0725   $252.07   $252.05   $252.10  
  Purchase #3 $16.67   11.104%   $18.521   $18.52   $18.50   $18.50  
  Purchase #4 $78.67   4.481%   $82.1952   $82.20   $82.20   $82.20  
  Purchase #5 $81.76   12.118%   $91.6677   $91.67   $91.65   $91.70  
  Purchase #6 $76.82   5.729%   $81.221   $81.22   $81.20   $81.20  
  Purchase #7 $137.69   9.172%   $150.3189   $150.32   $150.30   $150.30  
  7 purchase totals = $756.71       $811.2059   $811.21   $811.10   $811.20