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 $186.31   10.210%   $205.3323   $205.33   $205.35   $205.30  
  Purchase #2 $90.33   6.563%   $96.2584   $96.26   $96.25   $96.30  
  Purchase #3 $72.27   12.406%   $81.2358   $81.24   $81.25   $81.20  
  Purchase #4 $180.57   12.679%   $203.4645   $203.46   $203.45   $203.50  
  Purchase #5 $19.57   4.638%   $20.4777   $20.48   $20.50   $20.50  
  Purchase #6 $27.55   7.248%   $29.5468   $29.55   $29.55   $29.50  
  Purchase #7 $22.71   7.316%   $24.3715   $24.37   $24.35   $24.40  
  7 purchase totals = $599.31       $660.6870   $660.69   $660.70   $660.70