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 $25.72   9.317%   $28.1163   $28.12   $28.10   $28.10  
  Purchase #2 $25.87   10.839%   $28.674   $28.67   $28.65   $28.70  
  Purchase #3 $189.25   11.139%   $210.3306   $210.33   $210.35   $210.30  
  Purchase #4 $190.33   11.367%   $211.9648   $211.96   $211.95   $212.00  
  Purchase #5 $45.81   9.342%   $50.0896   $50.09   $50.10   $50.10  
  Purchase #6 $214.64   3.774%   $222.7405   $222.74   $222.75   $222.70  
  Purchase #7 $220.45   11.853%   $246.5799   $246.58   $246.60   $246.60  
  7 purchase totals = $912.07       $998.4957   $998.49   $998.50   $998.50