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 $11.39   12.437%   $12.8066   $12.81   $12.80   $12.80  
  Purchase #2 $83.76   6.675%   $89.351   $89.35   $89.35   $89.40  
  Purchase #3 $174.84   8.444%   $189.6035   $189.60   $189.60   $189.60  
  Purchase #4 $157.60   4.718%   $165.0356   $165.04   $165.05   $165.00  
  Purchase #5 $120.60   7.546%   $129.7005   $129.70   $129.70   $129.70  
  Purchase #6 $28.85   7.622%   $31.0489   $31.05   $31.05   $31.00  
  Purchase #7 $106.57   11.542%   $118.8703   $118.87   $118.85   $118.90  
  7 purchase totals = $683.61       $736.4164   $736.42   $736.40   $736.40