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 $29.93   13.284%   $33.9059   $33.91   $33.90   $33.90  
  Purchase #2 $197.65   7.601%   $212.6734   $212.67   $212.65   $212.70  
  Purchase #3 $118.38   8.624%   $128.5891   $128.59   $128.60   $128.60  
  Purchase #4 $5.51   3.593%   $5.708   $5.71   $5.70   $5.70  
  Purchase #5 $169.44   11.207%   $188.4291   $188.43   $188.45   $188.40  
  Purchase #6 $53.28   13.718%   $60.589   $60.59   $60.60   $60.60  
  Purchase #7 $51.89   6.212%   $55.1134   $55.11   $55.10   $55.10  
  7 purchase totals = $626.08       $685.0079   $685.01   $685.00   $685.00