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 $173.80   7.868%   $187.4746   $187.47   $187.45   $187.50  
  Purchase #2 $175.74   5.447%   $185.3126   $185.31   $185.30   $185.30  
  Purchase #3 $118.45   11.851%   $132.4875   $132.49   $132.50   $132.50  
  Purchase #4 $38.51   6.819%   $41.136   $41.14   $41.15   $41.10  
  Purchase #5 $13.80   7.818%   $14.8789   $14.88   $14.90   $14.90  
  Purchase #6 $30.83   10.468%   $34.0573   $34.06   $34.05   $34.10  
  Purchase #7 $47.84   4.654%   $50.0665   $50.07   $50.05   $50.10  
  7 purchase totals = $598.97       $645.4134   $645.42   $645.40   $645.50