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 $236.53   12.646%   $266.4416   $266.44   $266.45   $266.40  
  Purchase #2 $167.69   13.744%   $190.7373   $190.74   $190.75   $190.70  
  Purchase #3 $107.52   11.205%   $119.5676   $119.57   $119.55   $119.60  
  Purchase #4 $93.32   11.332%   $103.895   $103.90   $103.90   $103.90  
  Purchase #5 $177.50   13.458%   $201.388   $201.39   $201.40   $201.40  
  Purchase #6 $120.54   12.311%   $135.3797   $135.38   $135.40   $135.40  
  Purchase #7 $139.35   4.674%   $145.8632   $145.86   $145.85   $145.90  
  7 purchase totals = $1,042.45       $1,163.2724   $1,163.28   $1,163.30   $1,163.30