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 $25.98   8.435%   $28.1714   $28.17   $28.15   $28.20  
  Purchase #2 $6.26   4.736%   $6.5565   $6.56   $6.55   $6.60  
  Purchase #3 $8.87   4.818%   $9.2974   $9.30   $9.30   $9.30  
  Purchase #4 $7.61   5.731%   $8.0461   $8.05   $8.05   $8.00  
  Purchase #5 $139.37   7.490%   $149.8088   $149.81   $149.80   $149.80  
  Purchase #6 $164.64   12.156%   $184.6536   $184.65   $184.65   $184.70  
  Purchase #7 $245.43   6.110%   $260.4258   $260.43   $260.45   $260.40  
  7 purchase totals = $598.16       $646.9596   $646.97   $646.95   $647.00