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 $18.98   5.780%   $20.077   $20.08   $20.10   $20.10  
  Purchase #2 $202.94   12.732%   $228.7783   $228.78   $228.80   $228.80  
  Purchase #3 $132.63   8.648%   $144.0998   $144.10   $144.10   $144.10  
  Purchase #4 $11.38   6.742%   $12.1472   $12.15   $12.15   $12.10  
  Purchase #5 $109.26   5.581%   $115.3578   $115.36   $115.35   $115.40  
  Purchase #6 $97.71   3.356%   $100.9891   $100.99   $101.00   $101.00  
  Purchase #7 $92.56   3.744%   $96.0254   $96.03   $96.05   $96.00  
  7 purchase totals = $665.46       $717.4746   $717.49   $717.55   $717.50