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 $180.80   4.850%   $189.5688   $189.57   $189.55   $189.60  
  Purchase #2 $172.99   3.119%   $178.3856   $178.39   $178.40   $178.40  
  Purchase #3 $35.68   13.301%   $40.4258   $40.43   $40.45   $40.40  
  Purchase #4 $103.39   6.219%   $109.8198   $109.82   $109.80   $109.80  
  Purchase #5 $53.53   7.801%   $57.7059   $57.71   $57.70   $57.70  
  Purchase #6 $140.60   12.227%   $157.7912   $157.79   $157.80   $157.80  
  Purchase #7 $28.97   7.849%   $31.2439   $31.24   $31.25   $31.20  
  7 purchase totals = $715.96       $764.9410   $764.95   $764.95   $764.90