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 $7.48   11.515%   $8.3413   $8.34   $8.35   $8.30  
  Purchase #2 $17.36   5.559%   $18.325   $18.33   $18.35   $18.30  
  Purchase #3 $248.33   11.634%   $277.2207   $277.22   $277.20   $277.20  
  Purchase #4 $102.40   7.448%   $110.0268   $110.03   $110.05   $110.00  
  Purchase #5 $75.69   13.114%   $85.616   $85.62   $85.60   $85.60  
  Purchase #6 $100.57   13.109%   $113.7537   $113.75   $113.75   $113.80  
  Purchase #7 $187.42   6.847%   $200.2526   $200.25   $200.25   $200.30  
  7 purchase totals = $739.25       $813.5361   $813.54   $813.55   $813.50