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 $135.98   5.481%   $143.4331   $143.43   $143.45   $143.40  
  Purchase #2 $132.53   4.563%   $138.5773   $138.58   $138.60   $138.60  
  Purchase #3 $236.40   3.531%   $244.7473   $244.75   $244.75   $244.70  
  Purchase #4 $132.82   8.489%   $144.0951   $144.10   $144.10   $144.10  
  Purchase #5 $25.73   13.272%   $29.1449   $29.14   $29.15   $29.10  
  Purchase #6 $46.80   5.150%   $49.2102   $49.21   $49.20   $49.20  
  Purchase #7 $29.99   4.440%   $31.3216   $31.32   $31.30   $31.30  
  7 purchase totals = $740.25       $780.5295   $780.53   $780.55   $780.40