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 $71.80   11.435%   $80.0103   $80.01   $80.00   $80.00  
  Purchase #2 $161.79   11.461%   $180.3328   $180.33   $180.35   $180.30  
  Purchase #3 $60.84   10.186%   $67.0372   $67.04   $67.05   $67.00  
  Purchase #4 $23.60   6.842%   $25.2147   $25.21   $25.20   $25.20  
  Purchase #5 $31.81   12.687%   $35.8457   $35.85   $35.85   $35.80  
  Purchase #6 $5.57   5.843%   $5.8955   $5.90   $5.90   $5.90  
  Purchase #7 $100.79   6.826%   $107.6699   $107.67   $107.65   $107.70  
  7 purchase totals = $456.20       $502.0061   $502.01   $502.00   $501.90