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 $18.49   11.683%   $20.6502   $20.65   $20.65   $20.70  
  Purchase #2 $12.84   3.183%   $13.2487   $13.25   $13.25   $13.20  
  Purchase #3 $117.43   11.135%   $130.5058   $130.51   $130.50   $130.50  
  Purchase #4 $186.92   13.462%   $212.0832   $212.08   $212.10   $212.10  
  Purchase #5 $200.89   3.504%   $207.9292   $207.93   $207.95   $207.90  
  Purchase #6 $125.82   9.133%   $137.3111   $137.31   $137.30   $137.30  
  Purchase #7 $237.41   13.556%   $269.5933   $269.59   $269.60   $269.60  
  7 purchase totals = $899.80       $991.3215   $991.32   $991.35   $991.30