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 $249.32   13.561%   $283.1303   $283.13   $283.15   $283.10  
  Purchase #2 $42.62   3.816%   $44.2464   $44.25   $44.25   $44.20  
  Purchase #3 $118.94   4.484%   $124.2733   $124.27   $124.25   $124.30  
  Purchase #4 $125.68   13.515%   $142.6657   $142.67   $142.65   $142.70  
  Purchase #5 $240.96   8.552%   $261.5669   $261.57   $261.55   $261.60  
  Purchase #6 $61.35   9.662%   $67.2776   $67.28   $67.30   $67.30  
  Purchase #7 $36.49   3.830%   $37.8876   $37.89   $37.90   $37.90  
  7 purchase totals = $875.36       $961.0478   $961.06   $961.05   $961.10