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 $144.27   9.784%   $158.3854   $158.39   $158.40   $158.40  
  Purchase #2 $167.74   9.651%   $183.9286   $183.93   $183.95   $183.90  
  Purchase #3 $103.50   5.268%   $108.9524   $108.95   $108.95   $109.00  
  Purchase #4 $137.61   6.814%   $146.9867   $146.99   $147.00   $147.00  
  Purchase #5 $25.96   9.311%   $28.3771   $28.38   $28.40   $28.40  
  Purchase #6 $202.47   10.190%   $223.1017   $223.10   $223.10   $223.10  
  Purchase #7 $68.34   3.608%   $70.8057   $70.81   $70.80   $70.80  
  7 purchase totals = $849.89       $920.5376   $920.55   $920.60   $920.60