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 $8.37   4.722%   $8.7652   $8.77   $8.75   $8.80  
  Purchase #2 $16.42   6.335%   $17.4602   $17.46   $17.45   $17.50  
  Purchase #3 $70.32   10.473%   $77.6846   $77.68   $77.70   $77.70  
  Purchase #4 $148.83   4.696%   $155.8191   $155.82   $155.80   $155.80  
  Purchase #5 $87.86   7.351%   $94.3186   $94.32   $94.30   $94.30  
  Purchase #6 $74.51   9.805%   $81.8157   $81.82   $81.80   $81.80  
  Purchase #7 $184.55   12.655%   $207.9048   $207.90   $207.90   $207.90  
  7 purchase totals = $590.86       $643.7682   $643.77   $643.70   $643.80