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 $206.55   6.636%   $220.2567   $220.26   $220.25   $220.30  
  Purchase #2 $159.78   13.299%   $181.0291   $181.03   $181.05   $181.00  
  Purchase #3 $246.27   3.467%   $254.8082   $254.81   $254.80   $254.80  
  Purchase #4 $203.75   4.125%   $212.1547   $212.15   $212.15   $212.20  
  Purchase #5 $19.46   6.781%   $20.7796   $20.78   $20.80   $20.80  
  Purchase #6 $222.65   6.624%   $237.3983   $237.40   $237.40   $237.40  
  Purchase #7 $148.96   11.841%   $166.5984   $166.60   $166.60   $166.60  
  7 purchase totals = $1,207.42       $1,293.0250   $1,293.03   $1,293.05   $1,293.10