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 $52.29   12.845%   $59.0067   $59.01   $59.00   $59.00  
  Purchase #2 $152.58   13.201%   $172.7221   $172.72   $172.70   $172.70  
  Purchase #3 $205.39   8.462%   $222.7701   $222.77   $222.75   $222.80  
  Purchase #4 $54.94   6.308%   $58.4056   $58.41   $58.40   $58.40  
  Purchase #5 $126.50   7.253%   $135.675   $135.68   $135.70   $135.70  
  Purchase #6 $147.52   9.663%   $161.7749   $161.77   $161.75   $161.80  
  Purchase #7 $151.88   9.831%   $166.8113   $166.81   $166.80   $166.80  
  7 purchase totals = $891.10       $977.1657   $977.17   $977.10   $977.20