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 $79.80   10.173%   $87.9181   $87.92   $87.90   $87.90  
  Purchase #2 $59.98   11.816%   $67.0672   $67.07   $67.05   $67.10  
  Purchase #3 $190.92   8.831%   $207.7801   $207.78   $207.80   $207.80  
  Purchase #4 $39.29   12.254%   $44.1046   $44.10   $44.10   $44.10  
  Purchase #5 $81.49   11.811%   $91.1148   $91.11   $91.10   $91.10  
  Purchase #6 $31.25   10.294%   $34.4669   $34.47   $34.45   $34.50  
  Purchase #7 $95.54   6.645%   $101.8886   $101.89   $101.90   $101.90  
  7 purchase totals = $578.27       $634.3403   $634.34   $634.30   $634.40