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 $227.53   9.641%   $249.4662   $249.47   $249.45   $249.50  
  Purchase #2 $58.99   12.454%   $66.3366   $66.34   $66.35   $66.30  
  Purchase #3 $95.30   13.695%   $108.3513   $108.35   $108.35   $108.40  
  Purchase #4 $225.57   9.205%   $246.3337   $246.33   $246.35   $246.30  
  Purchase #5 $212.68   12.484%   $239.231   $239.23   $239.25   $239.20  
  Purchase #6 $138.95   13.353%   $157.504   $157.50   $157.50   $157.50  
  Purchase #7 $176.88   4.199%   $184.3072   $184.31   $184.30   $184.30  
  7 purchase totals = $1,135.90       $1,251.5300   $1,251.53   $1,251.55   $1,251.50