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 $28.97   3.598%   $30.0123   $30.01   $30.00   $30.00  
  Purchase #2 $79.73   6.332%   $84.7785   $84.78   $84.80   $84.80  
  Purchase #3 $103.56   7.796%   $111.6335   $111.63   $111.65   $111.60  
  Purchase #4 $128.80   13.408%   $146.0695   $146.07   $146.05   $146.10  
  Purchase #5 $138.66   4.630%   $145.08   $145.08   $145.10   $145.10  
  Purchase #6 $114.35   11.750%   $127.7861   $127.79   $127.80   $127.80  
  Purchase #7 $35.52   13.679%   $40.3788   $40.38   $40.40   $40.40  
  7 purchase totals = $629.59       $685.7387   $685.74   $685.80   $685.80