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 $17.70   5.238%   $18.6271   $18.63   $18.65   $18.60  
  Purchase #2 $27.33   11.507%   $30.4749   $30.47   $30.45   $30.50  
  Purchase #3 $167.29   11.388%   $186.341   $186.34   $186.35   $186.30  
  Purchase #4 $137.43   13.311%   $155.7233   $155.72   $155.70   $155.70  
  Purchase #5 $122.25   12.513%   $137.5471   $137.55   $137.55   $137.50  
  Purchase #6 $160.72   11.286%   $178.8589   $178.86   $178.85   $178.90  
  Purchase #7 $79.75   10.283%   $87.9507   $87.95   $87.95   $88.00  
  7 purchase totals = $712.47       $795.5230   $795.52   $795.50   $795.50