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 $120.30   6.845%   $128.5345   $128.53   $128.55   $128.50  
  Purchase #2 $25.52   7.812%   $27.5136   $27.51   $27.50   $27.50  
  Purchase #3 $95.25   3.129%   $98.2304   $98.23   $98.25   $98.20  
  Purchase #4 $76.31   10.344%   $84.2035   $84.20   $84.20   $84.20  
  Purchase #5 $11.51   12.837%   $12.9875   $12.99   $13.00   $13.00  
  Purchase #6 $103.98   9.602%   $113.9642   $113.96   $113.95   $114.00  
  Purchase #7 $98.42   13.722%   $111.9252   $111.93   $111.95   $111.90  
  7 purchase totals = $531.29       $577.3589   $577.35   $577.40   $577.30