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 $52.60   9.847%   $57.7795   $57.78   $57.80   $57.80  
  Purchase #2 $225.92   12.363%   $253.8505   $253.85   $253.85   $253.90  
  Purchase #3 $166.85   7.471%   $179.3154   $179.32   $179.30   $179.30  
  Purchase #4 $16.39   6.315%   $17.425   $17.43   $17.45   $17.40  
  Purchase #5 $213.31   3.451%   $220.6713   $220.67   $220.65   $220.70  
  Purchase #6 $59.37   9.176%   $64.8178   $64.82   $64.80   $64.80  
  Purchase #7 $173.85   11.191%   $193.3056   $193.31   $193.30   $193.30  
  7 purchase totals = $908.29       $987.1651   $987.18   $987.15   $987.20