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 $102.88   7.301%   $110.3913   $110.39   $110.40   $110.40  
  Purchase #2 $66.40   12.363%   $74.609   $74.61   $74.60   $74.60  
  Purchase #3 $37.89   3.194%   $39.1002   $39.10   $39.10   $39.10  
  Purchase #4 $102.94   4.698%   $107.7761   $107.78   $107.80   $107.80  
  Purchase #5 $18.51   13.873%   $21.0779   $21.08   $21.10   $21.10  
  Purchase #6 $194.61   11.618%   $217.2198   $217.22   $217.20   $217.20  
  Purchase #7 $61.40   11.708%   $68.5887   $68.59   $68.60   $68.60  
  7 purchase totals = $584.63       $638.7630   $638.77   $638.80   $638.80