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 $18.74   11.116%   $20.8231   $20.82   $20.80   $20.80  
  Purchase #2 $39.27   7.408%   $42.1791   $42.18   $42.20   $42.20  
  Purchase #3 $107.36   5.586%   $113.3571   $113.36   $113.35   $113.40  
  Purchase #4 $183.94   3.181%   $189.7911   $189.79   $189.80   $189.80  
  Purchase #5 $83.37   4.596%   $87.2017   $87.20   $87.20   $87.20  
  Purchase #6 $111.76   7.383%   $120.0112   $120.01   $120.00   $120.00  
  Purchase #7 $100.68   10.659%   $111.4115   $111.41   $111.40   $111.40  
  7 purchase totals = $645.12       $684.7748   $684.77   $684.75   $684.80