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 $20.42   7.346%   $21.9201   $21.92   $21.90   $21.90  
  Purchase #2 $79.70   9.328%   $87.1344   $87.13   $87.15   $87.10  
  Purchase #3 $226.74   13.566%   $257.4995   $257.50   $257.50   $257.50  
  Purchase #4 $177.35   10.423%   $195.8352   $195.84   $195.85   $195.80  
  Purchase #5 $241.28   6.482%   $256.9198   $256.92   $256.90   $256.90  
  Purchase #6 $126.62   8.322%   $137.1573   $137.16   $137.15   $137.20  
  Purchase #7 $20.27   12.561%   $22.8161   $22.82   $22.80   $22.80  
  7 purchase totals = $892.38       $979.2824   $979.29   $979.25   $979.20