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 $37.40   5.528%   $39.4675   $39.47   $39.45   $39.50  
  Purchase #2 $11.74   6.540%   $12.5078   $12.51   $12.50   $12.50  
  Purchase #3 $210.58   8.468%   $228.4119   $228.41   $228.40   $228.40  
  Purchase #4 $142.89   4.767%   $149.7016   $149.70   $149.70   $149.70  
  Purchase #5 $118.28   10.460%   $130.6521   $130.65   $130.65   $130.70  
  Purchase #6 $23.33   10.377%   $25.751   $25.75   $25.75   $25.80  
  Purchase #7 $63.25   7.211%   $67.811   $67.81   $67.80   $67.80  
  7 purchase totals = $607.47       $654.3029   $654.30   $654.25   $654.40