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 $247.69   6.736%   $264.3744   $264.37   $264.35   $264.40  
  Purchase #2 $100.75   9.819%   $110.6426   $110.64   $110.65   $110.60  
  Purchase #3 $125.47   7.661%   $135.0823   $135.08   $135.10   $135.10  
  Purchase #4 $236.31   6.718%   $252.1853   $252.19   $252.20   $252.20  
  Purchase #5 $5.75   4.479%   $6.0075   $6.01   $6.00   $6.00  
  Purchase #6 $217.95   4.149%   $226.9927   $226.99   $227.00   $227.00  
  Purchase #7 $181.54   3.404%   $187.7196   $187.72   $187.70   $187.70  
  7 purchase totals = $1,115.46       $1,183.0044   $1,183.00   $1,183.00   $1,183.00