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 $13.52   13.524%   $15.3484   $15.35   $15.35   $15.30  
  Purchase #2 $14.46   6.668%   $15.4242   $15.42   $15.40   $15.40  
  Purchase #3 $79.29   3.562%   $82.1143   $82.11   $82.10   $82.10  
  Purchase #4 $135.64   6.177%   $144.0185   $144.02   $144.00   $144.00  
  Purchase #5 $133.26   8.712%   $144.8696   $144.87   $144.85   $144.90  
  Purchase #6 $124.37   7.573%   $133.7885   $133.79   $133.80   $133.80  
  Purchase #7 $210.68   13.254%   $238.6035   $238.60   $238.60   $238.60  
  7 purchase totals = $711.22       $774.1670   $774.16   $774.10   $774.10