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 $7.84   10.796%   $8.6864   $8.69   $8.70   $8.70  
  Purchase #2 $104.93   8.465%   $113.8123   $113.81   $113.80   $113.80  
  Purchase #3 $227.53   6.414%   $242.1238   $242.12   $242.10   $242.10  
  Purchase #4 $38.46   12.760%   $43.3675   $43.37   $43.35   $43.40  
  Purchase #5 $30.37   9.576%   $33.2782   $33.28   $33.30   $33.30  
  Purchase #6 $35.50   11.781%   $39.6823   $39.68   $39.70   $39.70  
  Purchase #7 $87.64   12.367%   $98.4784   $98.48   $98.50   $98.50  
  7 purchase totals = $532.27       $579.4289   $579.43   $579.45   $579.50