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 $19.32   5.400%   $20.3633   $20.36   $20.35   $20.40  
  Purchase #2 $112.74   13.802%   $128.3004   $128.30   $128.30   $128.30  
  Purchase #3 $203.71   10.614%   $225.3318   $225.33   $225.35   $225.30  
  Purchase #4 $55.36   4.153%   $57.6591   $57.66   $57.65   $57.70  
  Purchase #5 $128.52   5.663%   $135.7981   $135.80   $135.80   $135.80  
  Purchase #6 $26.33   11.552%   $29.3716   $29.37   $29.35   $29.40  
  Purchase #7 $61.30   3.319%   $63.3345   $63.33   $63.35   $63.30  
  7 purchase totals = $607.28       $660.1588   $660.15   $660.15   $660.20