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 $43.63   12.109%   $48.9132   $48.91   $48.90   $48.90  
  Purchase #2 $247.81   10.293%   $273.3171   $273.32   $273.30   $273.30  
  Purchase #3 $165.37   9.586%   $181.2224   $181.22   $181.20   $181.20  
  Purchase #4 $123.96   12.370%   $139.2939   $139.29   $139.30   $139.30  
  Purchase #5 $87.67   12.446%   $98.5814   $98.58   $98.60   $98.60  
  Purchase #6 $139.75   3.183%   $144.1982   $144.20   $144.20   $144.20  
  Purchase #7 $101.68   13.167%   $115.0682   $115.07   $115.05   $115.10  
  7 purchase totals = $909.87       $1,000.5944   $1,000.59   $1,000.55   $1,000.60