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 $16.90   10.720%   $18.7117   $18.71   $18.70   $18.70  
  Purchase #2 $51.85   10.869%   $57.4856   $57.49   $57.50   $57.50  
  Purchase #3 $230.80   7.465%   $248.0292   $248.03   $248.05   $248.00  
  Purchase #4 $73.48   9.842%   $80.7119   $80.71   $80.70   $80.70  
  Purchase #5 $142.41   4.675%   $149.0677   $149.07   $149.05   $149.10  
  Purchase #6 $160.31   12.805%   $180.8377   $180.84   $180.85   $180.80  
  Purchase #7 $79.52   6.830%   $84.9512   $84.95   $84.95   $85.00  
  7 purchase totals = $755.27       $819.7950   $819.80   $819.80   $819.80