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 $203.51   13.496%   $230.9757   $230.98   $231.00   $231.00  
  Purchase #2 $164.49   8.191%   $177.9634   $177.96   $177.95   $178.00  
  Purchase #3 $163.61   11.158%   $181.8656   $181.87   $181.85   $181.90  
  Purchase #4 $246.29   6.390%   $262.0279   $262.03   $262.05   $262.00  
  Purchase #5 $25.56   11.694%   $28.549   $28.55   $28.55   $28.50  
  Purchase #6 $85.89   13.494%   $97.48   $97.48   $97.50   $97.50  
  Purchase #7 $226.80   4.251%   $236.4413   $236.44   $236.45   $236.40  
  7 purchase totals = $1,116.15       $1,215.3029   $1,215.31   $1,215.35   $1,215.30