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 $13.41   11.589%   $14.9641   $14.96   $14.95   $15.00  
  Purchase #2 $7.98   10.802%   $8.842   $8.84   $8.85   $8.80  
  Purchase #3 $250.95   10.824%   $278.1128   $278.11   $278.10   $278.10  
  Purchase #4 $194.54   9.496%   $213.0135   $213.01   $213.00   $213.00  
  Purchase #5 $147.69   3.469%   $152.8134   $152.81   $152.80   $152.80  
  Purchase #6 $98.84   8.171%   $106.9162   $106.92   $106.90   $106.90  
  Purchase #7 $79.36   13.516%   $90.0863   $90.09   $90.10   $90.10  
  7 purchase totals = $792.77       $864.7483   $864.74   $864.70   $864.70