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 $19.55   7.827%   $21.0802   $21.08   $21.10   $21.10  
  Purchase #2 $79.61   11.427%   $88.707   $88.71   $88.70   $88.70  
  Purchase #3 $212.45   6.634%   $226.5439   $226.54   $226.55   $226.50  
  Purchase #4 $187.78   12.587%   $211.4159   $211.42   $211.40   $211.40  
  Purchase #5 $141.87   4.295%   $147.9633   $147.96   $147.95   $148.00  
  Purchase #6 $133.70   12.144%   $149.9365   $149.94   $149.95   $149.90  
  Purchase #7 $37.31   13.374%   $42.2998   $42.30   $42.30   $42.30  
  7 purchase totals = $812.27       $887.9466   $887.95   $887.95   $887.90