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 $20.90   3.865%   $21.7078   $21.71   $21.70   $21.70  
  Purchase #2 $17.33   13.334%   $19.6408   $19.64   $19.65   $19.60  
  Purchase #3 $214.27   11.167%   $238.1975   $238.20   $238.20   $238.20  
  Purchase #4 $49.74   3.176%   $51.3197   $51.32   $51.30   $51.30  
  Purchase #5 $116.42   3.562%   $120.5669   $120.57   $120.55   $120.60  
  Purchase #6 $110.66   13.695%   $125.8149   $125.81   $125.80   $125.80  
  Purchase #7 $68.40   6.755%   $73.0204   $73.02   $73.00   $73.00  
  7 purchase totals = $597.72       $650.2680   $650.27   $650.20   $650.20