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 $19.91   5.349%   $20.975   $20.98   $21.00   $21.00  
  Purchase #2 $75.68   8.140%   $81.8404   $81.84   $81.85   $81.80  
  Purchase #3 $77.90   12.772%   $87.8494   $87.85   $87.85   $87.80  
  Purchase #4 $89.55   12.230%   $100.502   $100.50   $100.50   $100.50  
  Purchase #5 $139.79   9.443%   $152.9904   $152.99   $153.00   $153.00  
  Purchase #6 $207.51   6.467%   $220.9297   $220.93   $220.95   $220.90  
  Purchase #7 $50.51   11.792%   $56.4661   $56.47   $56.45   $56.50  
  7 purchase totals = $660.85       $721.5530   $721.56   $721.60   $721.50