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 $180.30   4.372%   $188.1827   $188.18   $188.20   $188.20  
  Purchase #2 $25.60   9.545%   $28.0435   $28.04   $28.05   $28.00  
  Purchase #3 $6.38   12.313%   $7.1656   $7.17   $7.15   $7.20  
  Purchase #4 $225.56   7.606%   $242.7161   $242.72   $242.70   $242.70  
  Purchase #5 $74.31   10.729%   $82.2827   $82.28   $82.30   $82.30  
  Purchase #6 $215.98   7.718%   $232.6493   $232.65   $232.65   $232.60  
  Purchase #7 $73.68   11.496%   $82.1503   $82.15   $82.15   $82.20  
  7 purchase totals = $801.81       $863.1902   $863.19   $863.20   $863.20