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 $176.78   6.197%   $187.7351   $187.74   $187.75   $187.70  
  Purchase #2 $78.87   7.535%   $84.8129   $84.81   $84.80   $84.80  
  Purchase #3 $106.82   10.778%   $118.3331   $118.33   $118.35   $118.30  
  Purchase #4 $84.31   8.488%   $91.4662   $91.47   $91.45   $91.50  
  Purchase #5 $15.28   6.168%   $16.2225   $16.22   $16.20   $16.20  
  Purchase #6 $92.43   7.144%   $99.0332   $99.03   $99.05   $99.00  
  Purchase #7 $40.43   4.238%   $42.1434   $42.14   $42.15   $42.10  
  7 purchase totals = $594.92       $639.7464   $639.74   $639.75   $639.60