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 $90.96   9.477%   $99.5803   $99.58   $99.60   $99.60  
  Purchase #2 $182.40   3.678%   $189.1087   $189.11   $189.10   $189.10  
  Purchase #3 $76.78   10.283%   $84.6753   $84.68   $84.70   $84.70  
  Purchase #4 $113.67   6.729%   $121.3189   $121.32   $121.30   $121.30  
  Purchase #5 $67.77   9.358%   $74.1119   $74.11   $74.10   $74.10  
  Purchase #6 $17.80   13.719%   $20.242   $20.24   $20.25   $20.20  
  Purchase #7 $97.77   12.551%   $110.0411   $110.04   $110.05   $110.00  
  7 purchase totals = $647.15       $699.0782   $699.08   $699.10   $699.00